CPU Trickery?? Tamron 90 SP Macro 2.8

So I'm noticing that due to CPU handing over aperture control to the body, this guy is only f/5.6 at the maximum "zoom"/minimum focal distance. Is it a simple correction of the effective aperture due to extension of the front element taken into account via CPU or what?

I have read somewhere it is but i thought it was only one stop.

I suspect it not being an internal focus mechanism being the fault, but virtually I don't see myself trying serious shots w/o flash or a ring light, so it's no big deal. Plus missing focus below like 8-11 is easy as heck, so it's not a huge deal. Now for some closer on portraits fact that ap starts to change as you close in is a bit annoying.

Do Nikon micros do that? Lets say, the 85 since its closer to this 90?

Most all macros do this. It's perfectly normal.

Just wanted to double check. It makes sense to make it idiot proof / quicker "mode change" but I'd still prefer it could do the 2.8 if I absolutely must take my chances to get more light in there.