Hearthstone's Chadd Nervig has confirmed that rerolling easy quests in Hearthstone had a logic change in the most recent patch.

  • A change was made so rerolling an easy quest has a 50/50 chance of giving you an easy or hard quest.
  • Hard quests will always reroll into an easy quest - that has not changed.
  • The hard quest pool used to be larger than the easy one, which is why you received harder quests more frequently when rerolling easy quests.

That solves that mystery!

Quote From Chadd Nervig

Hey all. We've seen a bit of discussion about daily quest rerolls, and I wanted to give you an update on what the difference you're seeing is.

Daily quests fall into one of two buckets. "Easy" ones are generally completable through most normal gameplay, like play 30 cards, and award 50g. "Difficult" ones ask you to do something directed, like play 20 Murlocs, but reward 60g.

A while back, we added safeguarding such that if you rerolled a Difficult quest, you'd always get an Easy quest (so when you don't want to do a difficult daily, you can reroll and guaranteed get an easier one). No such guarantee existed for rerolling Easy quests; when you have an easy one and reroll, we guess that you just want something different, so give you a different one out of the entire pool.

However, when we added all the different combinations of "play games on these 3 classes", that massively increased the total number of Difficult quests, which made "a random quest from the whole pool" skew heavily toward Difficult, which was unintended. In 17.0, we tweaked that to give you a 50/50 chance of getting Easy or Difficult, when you reroll an Easy, so that it's not so heavily skewed.

(Note, there are still a couple rare higher-value quests in each bucket, like challenging a friend. That hasn't changed.)