Villagers don't select random trades from their list of deals in the snapshot; they seem to have a pre-chosen order for what deals they have (although quantities and enchantments can vary). I think this is a bug because it's not listed on the wiki.

For example:
All Librarians will spawn with two deals: their first is buying paper, and their second is enchanting a book. After you've traded with them a few times, they will buy books, then sell a compass, then buy a bookshelf.

All Armorers will spawn first buying coal then selling a helmet. Then they will buy iron, then sell an iron chestplate , then buy diamonds, then sell an enchanted diamond chestplate.

Steps to Reproduce:
1. Spawn multiple villagers with the same profession (remember, some professions share outfits now)
2.Open their trading interfaces. You'll find that they have they have the same deals in the exact same order, with varying stack sizes and enchantments.