I get that "remembering specific things from old dreams" too. Like I'll remember a specific wall or something. It's quite weird.
Last night I was a lone soldier on an RTS, shooting barrels and blowing stuff up. Actually, I'm pretty sure I've had RTS style dreams before...
I also once dreamt I was being taught about games being art, as if I was at college, but there was no room; just the teacher, one of those massive-notepad-on-a-stand things, me and a few other students, and empty whiteness. We were asked this question; "Halo is all about fun. Do you like or dislike this? Give your reasons/Discuss", and my answer was "I dislike this because games are art, and art shouldn't all be about fun". The dream interested me, because this is the sort of answer I would've given when I was at college; stating academic facts and not really explaining anything.
As for why dreams are so weird, I think they're probably a loose thread of recent events and words you've taken in, often replacing the actual words with synonyms or showing things associated with them. I think that if someone managed to find a way to work out the actual coding of a dream, it would look like really bad translation.