think of it this way.. the other plants are feeding more "mouths" .. (aka extra axillary growth by comparison) ... so it's probably not growing "faster" despite most of the growth focused on 1 primary cola. if it has same total leaf area and same rootsystem, it's probably growing just as fast but concentrated in one spot... or if any of these things are lacking actually growing slower when you measure it all and tabulate it.
all sorts of variety in gowth patterns is possible... genetic depression can reduce axillary growth as well as overall vigorousness of growth. i think it's mistaken as "sativa" pheno at least some portion of the time at this point. inbred "outcropping" plants quickly turn to shit after just a couple generations.