I've never used the word "often".
It's not like: "oh my god no, I've won the first quarter, now I'm going to be dominated for the rest of the match!". Not at all. I'm simply saying that this happens more often than you'd expect in real world.
I'll try to be more specific. I don't think that the engine simulates any single play in the game, independently from the others. It would be too hard to generate vaguely predictable results in this way; I've seen many sports simulations on the market and I'm quite sure none of them works this way.
I think that the engine somehow decides how the game will end up right at the beginning. Can't say exactly what this means: maybe all the stats for each player are calculated, but that's unlikely, maybe some "trends" are calculated, along with an approximate final score, or whatever.
After that, the engine lets the game flow freely, and randomness plays a relatively big part. At some point, a check is made for consistency: if the game is drifting too far from the expected trend, the engine forces a correction.
That's it. I may be completely wrong of course, but I wouldn't care too much if a pattern seems to appear in a team's score, regardless of who's on the court and who they're playing against. I mean, better players will most probably have a better +/-, a higher stamina will surely let them perform better late in the game and so on, but in the end, the engine structure also plays its part.