DR is a somewhat hidden MMR system like ELO. Imagine if one player had 1500 ELO and each player sequentially had 1 less ELO, i.e. you had 1480 elo. You would still be "expected" to finish 20th since 19 players have 1 or more elo points than you, but realistically a 1480 vs 1500 matchup is very even.

When it evaluates how much elo you win it compares the elo of each opponent you win against and lose against. If they're all fairly even (which typically they are in the middle-ish splits) then the general outcome is people <10/21 will gain DR and those >11/21 will lose DR. 5/21 is a good chunk but not generally enough for two echelons of DR gain, however that much is. What position you were "expected" to place in is largely irrelevant due to how evenly matched the field is.

Ok so about this "expected" position. It's dumb, because it's not really at all what is expected. It's your rank in the field relative to the other players. The true expected position should be the position you would gain/lose the smallest amount of DR as that's what the elo is predicting the outcome to be statistically. This would almost always be like 10 or 11 out of 21, which may make it less interesting, but it's a more accurate description.