i always thought that it was done be height in past years.
I'm in the same boat about height/not being seen. I'm pretty tall for the girls, but there are a fair number of guys who are taller than me, much taller even, i think one of them was getting in the way at the photo, but i felt sorry for Roo, who was stood with me during it, hes a lot shorter than me.
The guys stood up at the front row were towering us this year =/