Under this behavior, if I understand it correctly, a post can have six flags: two spam, two inappropriate, and two off-topic, and not be hidden. Let's pretend someone posts a page of only "W"s. I consider that spam, so that's what I'd flag it as, but the Discourse description for spam specifically says that spam is an advertisement. So, some people might not flag it as spam and instead flag it as inappropriate or off-topic. Now let's say we have an argument -- do I flag it as spam, off-topic, or inappropriate? On other sites I've been on, spam is synonymous with off-topic. I could flag an argument as any one of those three and technically not be wrong. Then, anything you're flagging whether it be spam or something off-topic is inappropriate, so the inappropriate flag can be used for anything flag-worthy.
Flag categories are a grey area. The same post can fit into multiple categories, and if users don't agree behind-the-scenes and beforehand what they're going to flag the posts as, you can end up with posts past 3 flags up to 6 that aren't hidden. If 3 people flag a post, even for separate reasons, we can still conclude that the post probably isn't a good one, and it should be hidden. In the PM to the poster, the reason can just be something general. Posts with 3 flags of any kind should be hidden automatically.