Pet battles, as they stand, have a relatively high amount of randomness. I think there are two ways to see it now. First, pay attention to your fight logs and average the number of hits/miss vs crits. You'll find this can fluctuate wildly between battles of the same pets using the same abilities. The other way to tell that there is a significant random element is to pay attention to how much health your pet lost after each battle -- sometimes it's a lot, othertimes it's not that much -- and this amount can vary wildly. In an extreme example, your pet might go from 100% to 10% health one battle, and then go from 100% to 90% the next (against the same pet and level). That's a lot of difference, and this tends to be less than fun.
To fix things, Blizzard is reducing the randomness in pet battles through changes to the accuracy and damage output. Specifically they are removing the base 5% to dodge. This will serve to make things hit more often across the board.
Since the 5% base to dodge has been removed, without additional changes many abilities had a 100% chance to hit. To correct this, some abilities now have a base chance to miss, but in return their damage has been increased. Also notable is that each pet will still have at least one ability that will have a 100% chance to hit.
Some of the other highlights of the change coming up in patch 5.3:
- Hit chance will now be listed on abilities
- Crit chance has not been messed wth
- Higher level opponents will be easier to hit (miss chance per level down from 5% to 2%)