...We'd be told that when you walk it's on two parallel lines not one foot exactly in front of another so it's the same with front, back, cat stance etc.
Even walking on two lines...doesn't it take extra leg/knee strength to keep a uniform height as you go through the poomsae? For example if I'm in a long-front-stance, and I step forward into another long-front-stance, in order to keep my head-height uniform as I step, I need to be holding all my weight on one leg with a bent knee as I step forward. On some days that even makes my knees creak a bit.
So with regard to beginners, I think to some extent the problem is that they haven't yet developed the muscle memory to keep their head-height constant throughout the step, but in some cases also their joints and muscles aren't yet strong enough to do it well. Personally, not keeping constant head-height is something I always need to keep working on.
FYI Taekwondo Sine Wave