I don't think that's necessarily true. If I'm sparring full-resistance, I'm not formulating a game plan - I'm defending myself and trying to hit the other guy. That's the same thing I'd be doing if I entered a competition using similar rules. Of course, there are kinds of sparring I do that are about developing a game plan, but it's entirely possible for someone to spar in class in exactly the same mode they'd use in competition - whether it's grappling, striking, or mixed.
The distinction I see is one you've pointed out before: at a competition, I'm more likely to face someone I don't know (good thing) and a wider range of skills (good thing).