Well the idea is you bend over a stem to expose the new growth at the nodes lower down, more light gets to them which promotes more growth, hey presto new colas. If you keep going you end up with a wide canopy and keep tying down the taller stems to even it out.
It's a well debated subject for autos though as they are on a "timer", so over doing it could stress them and stunt the growth which would reduce yield and doing it into flower could do the same which could reduce yield.
As for yours, they're sill relatively young, will probably have a growth spurt soon so I wouldn't worry.
Some stains can grow different though, You could get two seeds of the same strain, one could be short and bushy and other a little stretchy. Think it just depends on there genetics and what they what strains they were bred from. Don't quote me on that though.