Little confused on TTL metering by my 20D

Hi all,
I am a little confused on this situation:

I have a Canon EOS 20D. I recently purchased the Tamron 18-270 VC lens and was testing to see if it suits my needs.

The test I did:

Kept my camera in 'A' mode, F 6 (or so) - kept it constant for both lenses

a) Attached my Canon 100mm F2.8 macro, metered at the target image. The camera metered and showed my required shutter speed to be 1/8 seconds

b) Attached my 18-270, zoomed in at 100mm, metered at target image. The camera metered and showed my required shutter speed to be 1/3 seconds

I am a little confused. I would have imagined that since the aperture and light conditions are the same, and the selected focal length is the same, I should expect similar metering results. This should not be a function of the lens, unless it is really dirty, obstructs light or anything, which is not the situation in my case.

Thanks
arjun

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http://www.arjunphotography.com

I type faster than I think - and no, I am not a fast typist, just a slow thinker. So please excuse grammatical errors and typos.