I guess it's really just the height, I cannot explain it in any other way. I will show you my training from last year for Polh (18yo, 188cm) and Zorin (20yo, 183cm):
1. DR for SF/PF: Zorin pop in JS, Polh pop in HA and IS
2. JS for SG/SF: Polh pop in JS
3. DR for PG/SG: Zorin pop in HA and DR, Polh pop in DR
4. DR for PG/SG: Zorin pop in JS, Polh pop in JS and HA
5. JS for SG/SF: Polh pop in DR
6. DR for PG/SG: Zorin pop in JS
So out of 6 trainings, five pops for Zorin and seven pops for Polh. I guess it's really just the height...