I don't have my book with my at the moment, but I'm pretty sure nest0r is right. Radicals are what the Japanese actually categorize kanji by, and are part of their teaching process. Primitives are an artificial thing Heisig made up that occasionally but not consistently matches up with the accepted radicals (try look up a kanji by radical. You'll quickly find a lot of your primitives don't exist, or at least not in that form). While Heisig's primitives do occasionally coincide with actual radicals, it's only out of convenient happenstance; he doesn't teach radicals, as a general thing.

I think the primitive/radical distinction you made for the top stroke of 天 was self-fabricated.. I don't believe he ever mentions radicals outside the introduction. I don't remember if he did have any guides on it, but I personally just remember it on a case by case basis. If you need to, just incorporate the differences into your mnemonic. Turn a slanted heaven into a fall from heaven or something (whee, slide!)

Ok I get it now I guess, they're just more radicals combined, thanks. I had a hunch but some of the stuff he said was misleading me to believe that.

I misewell just call it early death from 夭  I suppose, since it seems to match it.