The longer answer
The post you stand on decides what the bird looks like. From P1 on the far left, most clays the trap throws move away to your left or straight out in front of you — left-quartering angles dominate. From P5 on the far right, the mirror image: clays favour the right. P3, in the middle, gives the most symmetric set of angles — extreme lefts to extreme rights — and sees every angle the trap can throw.
A round of trap is 25 targets — five shots at each post, then everyone in the squad slides one post to the right (P5 walks behind the line to P1) and shoots their next five. By the end of the round you've shot the trap from every angle the field offers, which is why the same five-target sample at each post is enough to score the day.