I don't think it's that uncommon to be on the 99th percentile for one test and lower for everything else, puffin, especially when you're talking about a 2e kid.

The test that I think you could most easily "prep" for based on observation is probably Information. For example, there was a question about who a particular historical personage was that my DD got right (probably because she happened to have received a picture book about him/her as a gift from a family friend a few months before the test). I agree that Vocabulary could also be problematic. (I'm trying to speak in general terms here so I am not guilty of revealing test questions.)