Something else to note about one's level dropping: it's not always a function of lower skill. It always feels bad to lose against someone who's level 2 or 3 or even 10 if you're level 25. But remember that those levels are partially a function of variance. If you're level 25, and you lose to a level 5, your base skill number goes down, but your variance also increases; this has a twofold effect on your level, as level is the difference between base skill and variance.
I'm not sure of actual numbers, but it looks like during my recent six-level drop, 2 or more levels can be attributed to increased variance. It's funny because it feels like most of my recent wins have been against similar-level players, but my losses against newbies! The worst, of course, is losing to a level-0 by obvious luck (they get 5/2 on a Witch board, you Potion shows up on turn 5 or with two Coppers on a Familiar board).
But, some of Dominion is always luck.