Looking in my own gmail spam folder, some emails have this different message:

"Why is this message in Spam? It contains content that's typically used in spam messages."

I'd guess this one means they inferred that it's spam based on the actual words in the email, whereas "It's similar to messages that were detected by our spam filters." might mean that it has a small edit distance from messages that have been explicitly marked as spam by many users (though it's definitely vague).

If that's the case, then it seems like displaying one of these two messages is pretty much all the information they could possibly give without actually saying exactly how the spam filter works.

I was thinking the same thing. It would be really interesting if it went into details about which aspects of the message were considered spam-evidence, and how strongly they weighed in the decision. That might not be a particularly useful feature, and Google might consider the specifics of its spam filter to be trade secrets, but it would sure be interesting.

I guess lets you know it's Google who threw it in the spam folder not one of your filters.

Seems like that would be giving ammunition to the adversary.