My actual program is similar to Neddy's: Chest+Calves / Back / Legs / Shoulders / Arms+abs.
However, I often can't make it 5 days so if I only have 4 days, it goes to chest/tri, back/bi, legs, shoulders and 3 days it would go to chest/shoulders/tri, back/bi, legs.
As for why.. well generally I do chest/tri and back/bi together because the arms are already warmed up and I find it easier on the joints (as less actual weight is required to hit failure - query whether this means less growth). The only thing I will switch up is whether to switch to chest/bi, back/tri.
That's what my first PT told me years ago and I've generally stuck with that program.
I generally don't do shoulders/bi's together because I've read that fatigued shoulders can lead to injury when you train biceps immediately afterwards.