My wife and I noticed similar differances in her baby Pbass. Eventually the growth rates tend to even out at least in my expierance. My silver aro will seem to 'stall' for a while and grow noticably thicker just before going into a growth spurt that usually adds another inch or so to her length.
Niether one of us is a strong proponant of artificially high growth rates or 'power feeding' as it's often called to get 'instant' monsters. We know from feeding our dogs and wolves a raw diet that they tend to grow slower but have a MUCH higher resistance to health problems and on average a much longer life as a result. Granted fish are NOT wolves but we feel the basic princaple is just as sound...
It's interesting that you mention that. I had some juvie altifron geo's not too long ago, and one grew faster than the other, then the smaller one somehow caught up.