Tamron 90mm f2.8, fixed aperture ? ?

I've ordered the Tamron 90mm f2.8 after reading many excellent reviews about it. However, while browsing through some more reviews i noticed the below similar comment in a couple of them which sounds really strange:

"The maximum aperture drops as you focus closer, and at minimum focusing distance, where many will want to work to get the true 1:1, it is all the way down to 5.6"

How can this be? I've always thought that a fixed aperture lens is a fixed aperture lens, meaning that no matter of the focal length you always have the f2.8 i.e. available.

Would appreciate an explanation on the above if possible. I know this lens is excellent because i've seen noumerous images so i am not worried about my choice, but i would really like to know if this can be true and why it happens

Many thanks in advance for all kind input on the matter.