Combination of several different things.
1. Marok's work on trying to define stat weights - mmo-champion.com /threads/1892699-Protection-legion-Discussion/page71?p=42198650#post42198650
2. More from Marok - mmo-champion.com /threads/1892699-Protection-legion-Discussion?p=42304937#post42304937
3. The first two points were pre-IP nerf. The IP nerf, IMO, hit versatility harder than Mastery (Mastery not only increases IP, but increases block which wasn't affected)
4. Review of recent Warcraftlogs rankings for Prot warrior EHRPS. Most of the top ranking warriors over the past 2-3 weeks have been stacking mastery>vers=haste. That's my general observation but I don't really have the time or desire to try and extract the data to put together actual numbers. All of those are based on player skill as well (there are some high rankings by people with more haste, and even high crit) but the trend seems to be people stacking mastery in the high rankings
Had to break the links because I'm not old enough to post links yet