The quest drop consists of uniques/sets/rares and charms/jewelry/runes. There won't be gold/potions/keys/other junk or magical items in the drop.
However, it is still possible to get failed set/unique items. For example, hell Baal tries to drop unique Archon Plate and set Cryptic Axe in his quest drop. Those items do not exist, so you will get rare Archon with triple durability and magical Cryptic Axe with double durability.
If you got 4 magical items in a quest drop, you actually got 4 set items which do not exist.
Act Boss quest drops (the first time you kill them) exclude junk (arrows, potions etc) and blue (magic) items. That means you can only get "goods" (runes/gems etc) and unique/set/rare equipment pieces. However, since the drop mechanics first choose the base item type, then the quality (unq/rare etc), if it rolls a set with a base item that doesn't have a set version, you get a double durability blue. That seems to be what happened to you. Interestingly, that phenomenon is more common than you'd think. Almost like Baal drops more blues than greens in his quest drops!
Edit: sigh, pwned again. You see, another strange phenomenon: No replies for over half an hour, but one (usually more) within the 5 minutes since I open the thread till I click the post reply button!