I have had a Kenwood KAC-929 forever, and I just picked up a second one for my other car.

Well, I tried it out in the car that already has the same one in it, with the same exact settings, and it has noticeably less power!

Was my old one a fluke? Maybe they got crappier as the serial numbers went up?

 

They shouldn't be all that different. Ya, there are going to some that are a little louder, but it should be all but impossible to notice. Sounds like something may be up with the second amp.

 

return it and try it again my friend got a second amp that i installed without the first one to start and it sounded a lot quieter then i stock the first one back on it and it was way louder and noticable id say return it and get another.

 

I had that happen to me... I was borrowing my brother's Pioneer760 to run some JL10W0's, it sounded pretty good and they are cheap so I bought one of my own. The new amp was WAY louder and smoother sounding. That didn't seem too suspicous considering it was newer. So my brother exchanged his amp for a new one, and that new one wasn't even as good as the very first amp.

that just shows inconsistancy in 3 of the same amp, with the same audio system.

 

Its not common for amps to have "noticably different power". BUT, on lines like pioneer, kenwood, alpine, and other mass produced amps, you will have this occur more often due to quality control.
Brands that have a more specialized line and better Q.C. like JL, Rockford (old), Kicker, Directed (old), and alot of other small specialized amp manufacturers like R.E. will have less variations between amps.
There is yet another twist to this scenario. For an example Rockford fosgate made an amp designated the 200a2. During the same run of amps there was another labeled 225a2. Internally they were the exact same amp, but there was a difference of about 200 watts between the two. This was done intentionally by RF, and the 225a2 was sold to individual dealers while the 200a2 was a general retail market amp. There are other occasions and tricks built into alot of amps that allow you to "bump" the power output. i.e. cutting two trace jumpers on an old RF power 800a2 to allow 1ohm operation and bump output by 500 watts.
Back in the day, dealers got first dibs and special runs by manufacturers to steer clients toward the independent dealers. This is what really set shops apart from retail outlets. Of course they also knew the tricks that made a big difference in the way a product could perform. Unfortunately, this doesn't really happen anymore.
SO... yes there can be a difference between these products, but it is all coincidental, or accidental on the fore-mentioned "mass production" amps.