Normally, we really don't care much about the exact file size. Mostly, we just want it to look decent, but be as small as practical for web posting.
But the very same program has other parts to it that allow for very fine increments of compression, and moreover, the size of the file displayed ends up being reasonably accurate in those other sections of the same program.
To me, it's just bad programming to end up with a calculation error like that. If we were estimating something based on hand calculations, it'd make sense to take shortcuts and allow for a certain amount of "slop". But this is a computer program. We've got enough computing power in our PCs to do the really hard stuff involved in photo editing. There is no reason not to take advantage of that horsepower and make the file size calculations accurate.
As a programmer, I just find it somewhat odd that you'd put code in one part of a program that works well and then go to the trouble of creating new and different code for a different part of the program that doesn't work properly. I really believe that these different parts of the program must have been written by different people and that they do not, for some reason, share bits of code between each other.
When I write a program, there is no reason to ever take shortcuts in the coding that results in inaccurate calculations. Since you only need to write the algorithm once, it pays to do it right that one time so that the calculations will be done correctly over and over as people use the program. This is not a place to worry about saving steps in the calculation because, as I say, you only need to write it once. After that, the computer is basically a slave. It will dutifully perform those tedious calculations over and over without complaint. So shortcuts that we'd use when hand calculating things have no place in a program IMO.
And two points:
While we normally don't worry about file sizes much, in this case, with a restriction of 150K, and the imperative to make our shots look as good as possible, most of us take a lot of pride in every detail. And one of those details ends up being the compression of the file. It's just one of the many aspects of this challenge. And it's part of the fun.
Next, I've seen you question why we'd care about having the EXIF intact. But there are two good reasons for it.
First, PBase automatically reads the EXIF in a file and inserts the date for the shot and some of the camera and shooting information. Since, to qualify for Eligible, the shot must have been taken within the challenge period, and it must have been taken with a Canon DSLR, having the EXIF intact automatically fills those fields out for us. So it's a time saver.
Second, since this is a photo exhibition and challenge for fellow photographers, most of us are really interested in how various effects have been achieved. This is not just an art exhibit aimed at the general public who might rightly have no interest in the technicalities of how a shot was created.
One of the best things about these challenges is that we learn so much from each other. People often go to a great deal of trouble to post very well made tutorials describing exactly how they've done some of their shots. If someone asks, they usually get a very amazing level of explanation.
And even though I grew up shooting film when we had no EXIF, the fact remains that it IS interesting to me to be able to see what settings were used for a shot. Don't imagine that the participants here are fools who think that they can just use the same settings as they see for some shot to get the exact same effect. Instead, think of how having every bit of data can still be of interest.
I might look at a shot and think: "Thats got great DOF and yet everything looks very sharp. If they used a very small aperture, I'd expect more diffraction issues, but here, we've got great DOF and sharpness. Gee, I wonder what aperture they did use."
Or I might think: "That shot looks very clean with no noise. Yet the lighting seems difficult. I wonder how they did that?" And then I right click the shot, view the EXIF, and find that they shot at a very long shutter speed. So then I know that they did use a low ISO, but must've used a tripod and long exposure to get the effect.
I could go on and on. But the fact is that there are a lot of reasons to want the EXIF data. And particularly for these challenges, people greatly appreciate being able to know everything they can about the shots. This is not a website just showing off photos to non-photographers. Nor is this me emailing a shot of my brother to my aunt. It's a set of galleries aimed, in great part, at other photographers in a spirit of learning and teaching. So if I don't include the EXIF, I feel like I'm shortchanging our fellow challengers. I want to see the EXIF, so I figure other challengers may too.
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Jim H.
..sorry John, I feel for you man.. 
( I am celebrating and talking as if I came in first ..
, but thats how happy I am.. )