"Congressional approval rating" is a dumb and misleading stat and I hate that it's reported on so much, especially in contexts like comparing to to e.g. the President's approval rating.
Maybe you love your representative and Senator(s) but your party is in the minority. So, you don't approve of Congress.
Maybe ditto on the liking your reps, but Congress is always doing really stupid shit even when your side's in charge (like the goddamn own-goal "debt ceiling" bullshit) simply because they can't rally enough support to make that junk stop, so you don't approve of Congress. Or maybe your side controls both houses and the Presidency but still can't do jack-shit because of Senate rules, or because some other Senator or two (not yours) in your preferred party like playing both sides to advance their state's interests at the expense of everyone else—so you don't approve of Congress.
Maybe you like your reps and your side is in charge but you've really taken anti-politician stand-up jokes to heart, or follow media that highlights and exaggerates all the stuff you hate about reps on the "other side" even when yours is in charge, so you tick the "disapprove" box.
And so on.