"If we are crunching the numbers, though, it seems like the flip side is much much more common, i.e. people doing things to benefit themselves under ostensibly altruistic motivations. "
Well, this is typical behavior, we all know that.
That is why I was puzzled by the phenomenon when everything happens in reverse.
But a much more common situation is when people clearly indicate and voice their position: they enter into interaction with others for the mutual formal benefit of both parties (nothing personal, it's just business), or for altruistic reasons.
But what my observation reveals, and what is probably even more common, is that people interact very often simultaneously satisfying both needs - for the good of others (you can call it altruism) and for their own good
If we talk about rationality in the broad sense of the word, as it is used on lesswrong, and not about rationalization in the narrow sense (and in general, these are different words with different meanings, although one-rooted), then rationality means "how a person should think and act in order to maximize the benefit of all, and the concept of "all" includes all people equally, and therefore himself"