Same experience here, PHL is a nightmare for carry on bags (and many other things, but since this post is about carry ons..).
The thing that kills me is when you encounter these crazy agents, apparently someone knows how to get through, because, without fail, there's always a ton of people at the gate with oversized bags.
This is doubly frustrating because I have an "old" Tumi bag that does not fit in the sizer. When I see them checking, I try another terminal; if that doesn't work, I'll give up the bag and hump my laptop bag alone (which weighs about as much as a small black hole; it's a nightmare when I lose the wheel bag to put it on) through the terminal. To get to the gate and find that, not at all surprisingly, about 1/2 the FC cabin has the EXACT same bag that I just had to give up. Unfortunately, they are getting better and better at seeing that my bag is 2" over and I guess I'm going to have to break down; spend another 1000 bucks or so and get the bag that's identical but 2" less wide.
The thing that truly pisses me off about this (even more than the fact that 1/2 the FC cabin has the same damn bag) is that the Tumi carry on is sized perfectly for the overheads on the 737 and 321 that fly most AA and US domestic routes. Fits in wheels out without a problem. Then you see people carrying on this crazy shaped stuff (that I supposed they crammed into the sizer somehow) that has to go in sideways and takes up a ton of space compared to my bag.
Sorry, just had to get this off my chest, my home airport is great with bags, so I never have this problem on the outbound, only on return. But hey, at least it's not British Air, they are even more bag crazy than the PHL airport; they tried to take away my laptop bag AND my Tumi carry on during a recent flight.
The funny thing; when they are really good at getting the bags, you get on the plane and the darn overheads are near empty (because so few bags really fit in that sizer). It's like job security for the "throwers" to make sure they get more bags to handle. I suppose AA gains something by forcing me to check a bag (which is free for me) but I fail to see what it is.