The interviews cover a lot of potential material in depth and I think the perspective is that not everyone works with all of those topics frequently and may need a refresher. Since Google knows what skills they want the interviewees to have, they share that information so the interviewee can round themselves out so that Google can identify their potential fairly. Like I said before, my experience was that Google tried very hard to avoid turning people away because they accidentally hit a weak spot or quizzed them on some bit of trivia.

For me, the preparation was primarily:

* honing the skills I did have to make sure I was comfortable using them in a tense situation

* refreshing and re-honing some things I hadn't seen in a while (some since college)

* filling in some gaps of knowledge (so I could connect the dots in an in-depth explanation better)

* preparing my thought process for the types of questions I'd have

The last one is subtle but important. If the interviewee has a good idea of what is expected of them, they can converge on a good path quickly and avoid going down rabbit holes or wasting 15 minutes trying to get on the same page as the interviewer.

As a side point:

> doesn't that mean your current skills aren't a good fit

Don't they really care about your skills at the time of being hired (which they approximate by measuring them at the time of the interview)?