Hm.. I can only think of one reason why your screecher is holding agro better, is cause the other pets you had used abilities that were instant and used up most of the pets focus so it could not use growl a lot. Pet dps doesn't play a huge factor into holding agro, its the amount of growl it does that matters. If your pet does only 1 growl and no more you can expect to pull agro off your pet. Try with another pet of the same lvl as you, except only have growl on and everything else off, I'm sure you'll notice that it'll hold agro just as good.
Do a count on how many lightening breaths vs bites with having growl/breath on in one fight, and growl/bite on in another against the same lvl mob with your screecher. Pretty sure growl/bite combo > growl/breath for dps.
Edit: Also your pet has to have the max lvl of growl it can learn, since it doesnt cost any training points it easy to upgrade. A pet trainer teaches you the various ranks of growl and other things. Growl is like a sunder, it generates hate, growl isn't a taunt contrary to what many people think.