Basically, bbers and PLers have a different agenda with respect to bench, and we use different forms to suit our goals. A bber may be trying to work his/her chest, and a PLer will be working on bench pressing more.
The primary mover in a BB bench is the chest. You unrack the bar, elbows go out wide, directly under the bar, and the bar descends to the chest. Then, if you don't have bad rotator cuffs, you press the weight back up using your pecs and shoulders, etc.
The primary movers in PL bench are the tris, lats, and shoulders. The bar descends to the xyphoid, with arms close to the body (45 degrees out for a wide/extra-wide grip). If you put your arms to your side and bend your arm at a 90 degree angle, as if you were a cowboy shooting imaginary guns, this is the plane your bench would travel through....with your arms next to your body and locked at 90 degrees, swing them back and forth at the shoulder. This is the bottom of the PL bench motion. This form limits shoulder rotation and recruits muscles that will allow you to ultimately press more than your chest would.