I've seen this same thing a lot with my af's. From what I can tell, if you dont switch the lights to a flowering spectrum early enough, they get stunted and won't ever achieve their full height. Although, that isn't to say that some are just smaller phenotypes in general. My suggestion is that you should switch to a flowering light at the very latest by the time they start showing sex. I usually put mine under flowering after about the third week and just watch them closely so I can catch any males before they release pollen. Also, I think LR1's suck in general and you should definitely go to LR2's or easyryders/short stuff or whatever.
It is okay to have them go past 8 weeks, but they usually don't grow much more past that.