I keep getting suggestions to increase my Haste gems/enchants, but the Optimal gearing strategy clearly shows I should be pushing Mastery instead. How do I correct this?
You don’t - as that’s not an error. Gearing in WoW has become extremely complex and the stats of the best possible combination of items you own can differ substantially from generally stronger setups.
For your currently owned items, it’s best to gem and enchant haste. Generally speaking you could get better results if you had more items with mastery though.
Another way to look at it is that Mastery from gear usually comes in larger amounts than gems/enchants do…