I don't know about Google getting ready to do a major UI overhaul (again), that would firmly be in rumor territory, but it is true that different groups within Google lead development of different applications. GMail is handled by one group, the Drive application is handled by another, and so on. Some of those widgets haven't had their UI changed in a while, which is partially responsible. It is also true that not all of the Google applications follow an identical version of the design guidelines, which does lead to a little internal inconsistency.
That may partially be to show third party developers the different kinds of options they have, if intentional, or, if unintentional, might just be due to the disparate groups of developers at Google maintaining different applications separately. It may also be due to the very different purposes each of those widgets has, and what data, options, and features those widgets are meant to display. To be honest, it has never really bothered me personally, as the Holo theme is relatively flexible, aside from the heights. I do think the heights should follow the design a little more closely, since there is a guideline specifically for that.