What you are likely running into is the higher rated players are playing more principled chess and you are more comfortable in those games, finding it easier to make plans.
Some of the lower rated players are likely playing in a manner that you are finding more difficult to respond to, especially in the faster time controls.
I almost always find that my estimation/evaluation of positions doesn't really reflect what is actually going in (as it comes to computer evaluations at least).
I'm sure if you really analyzed the games, the mistakes would be there and they aren't playing close to any GM level. And if they were, they wouldn't be 1500 rated for long.
I just don't understand this at all. Whenever I play against a 1500 player in blitz (with not much time to think) they almost always play like GM's! But whenever I play 1750's in standard (with plenty of time to think) they play completely normally. I'm talking about my opponent's moves, not my own. Of course I play to a lower standard in blitz, but my supposed 1500 rated opponents seem to play suspiciously quickly and accurately with no mistakes. Heck, whenever I watch GM blitz games on live chess even they make mistakes! What on earth is going on?
It's so unbelievably frustrating. This site really gets on my nerves sometimes.