Yes, the fact is that the ELO system is based on an expected score of .75 for 200 points difference. .75 means e.g. (75% win, 25% loss, no draws) or (50% wins, 50% draws, no losses).
So it's no surprise that you occasionally beat somebody even 300 points higher than you. It won't happen regularly, but you don't need a miracle or something.
For what you're saying in playing in a higher class OTB, I believe you're correct. The fact is that with chess there's also the draw. If you play somebody who is 100 points lower than you, and this guy is hellbent in getting a draw, most likely it is gonna be a draw in the end (especially if you're approaching 1800 and up). Your rating will be negatively affected, but with no "fault" of your own. Indeed occasionaly you will lose against weaker players because of a blunder, and these rare losses will hit your rating hard...while your wins against similarly rated opponents won't push your rating up as much as a loss push it down.
I'm gonna test this myself in my next tournament OTB. I'm gonna be playing in the under 2000 class instead of the under 1800 where I played last time. I am positive I am not a 1800 player (even if my last OTB performance was), but I fear I might encounter some 1600s that wants to draw and wrecks my rating.
So I'm gonna play against stronger opponents and try to score just a couple of points.
And all ratings consideration aside, playing against stronger people should really improve your game. You'll see where you do wrong, and what you did right. Hopefully your opponents will also give you some suggestion.
When I first began playing chess, I was under the incorrect assumption that the stronger player virtually always beats the weaker. While this idea may be more true as the time controls get longer, most of us internet players play in time controls no slower than g/30.
On another chess site, my 'seek' rating hovers around 1650. I'd stick an open seek out there for a g/20 or so, and play whoever accepted it (with only a restriction of players 200pts below my rating excluded). I found that my rating would slowly decline the more I played, because weaker players would accept the seek 90% of the time. My seek rating is now at 1600 on that site.
In fast chess (and maybe all time controls), the truth is, sometimes you just lose. An oversight, a bad game, whatever. It happens, sometimes you just can't win. And if you're ALWAYS playing weaker players, your rating will shrivel, because the few losses will outweigh the many wins.
I decided to start playing in an auto-pair pool of g/15 (on the same site) and low and behold, after 30 or so games, my rating jumped to 1800 (still with an upward trend). I believe the reason why is because half of my games are against stronger opposition than myself (sometimes a difference of over 300 pts!). Of course the difference could be simply that the ratings in that particular pool are inflated... BUT
If you take anything away from this post, consider playing in a rating section higher than the one you're supposed to next time you play otb. You may be 'plateuing' in a particular class simply because you continually play weak opponents, and inevitably lose a game. This masks your 'true' accurate rating and portrays a weaker one. You may find that you can maintain a good win percentage against stronger opponents, which not only will boost your rating, but will boost your understanding of the game.
Anyway sorry for the long post, I'd love to hear your thoughts on the matter. cheers
Jack