It's frustrating, but really, how can you solve it?
We all run into this, but honestly its pretty simple to overcome. If the cap is just a few under current numbers, then try logging off, but without closing the AH client log right back on again. Check the numbers, rinse, and repeat. As people log off, the numbers drop steadily and within a couple minutes you'll have a slot. (And that's how the numbers can pop up again...if 5 people happen to log on when the server is seeing "198/200" they all get a slot reserved for them. So, if you don't check repeatedly you'll see the totals go from 201 to 200 to 203.)
If the low arena totals are far from the high arena, and the low arena is right on the border of being identified as critically lower, you can run into another problem. Say you get booted with the two being at 300/300 and 201/350. (Just making these numbers up.) If the low arena/high arena proportions fall too far out of whack the system will apply more pressure to the high arena by cutting the allowable slots, so things might suddenly change to 299/250 in high and 199/425. This makes it look like there'll be a long wait for the bigger arena, but usually what happens is that the limits will pop back into line within a few minutes...so you'll see 299/350 and 203/450 if you just keep trying.
At worst you can go to another arena for a sortie, and expect things to be reasonable in your preferred arena when finished.