Also to throw another *angle* to the mix (that's a pun, you'll see why in a moment):
The body position can greatly effect how people Overhead Press and how they Seated Press. Typically people don't Seated Press with a fully upright Bench, in fact I rarely see it. What I normally see is a slightly inclined Bench, couple that with some back arch and you get something which is nothing more than a high incline with a big emphasis on chest and front delts. A proper standing Press forced you to Press more upright and places greater stress across the shoulders, I know people backbend but rarely to the same degree as with an inclined Bench.
Here for example, nothing against this fella in the video (except the ghastly music) but the lean back is horrible:
As a result it's another reason why people generally seated press more than they can standing press.
See, wasn't it worth reading that so the pun made sense!