My guess is there was probably one design committee that decided that things needed to look different, because the "future is now", we can't remind people of older versions of windows because visually that isn't progress, and we can't be reminiscent of the "bad old days" when things crashed often.
Then after that committee makes it's decree, everybody who has control over their own little gui fiefdom each decides "nah.. that looks terrible and we aren't going to be cohesive, so lets do our own thing over here."
And so, everything looks and acts a little different. Since not everybody got on board with the change, the original committee digs in their heels and says, "Well it obviously doesn't work because not everybody is on board with OUR design."