User blog comment:Emptylord/Archived/New Boots and Enchantments (2015)/@comment-24795478-20150515124745/@comment-4022742-20150520024558

If mercury's and the mastery would stack multiplicative, then it'd be 15.6% magic AoE and 15% physical AoE. It'd hardly be worth getting the mastery at all. But if they'd stack addatively then the combined 19% reduction would be too strong (something they removed from gragas).

10% addative stacking seems like a legit compromise, 10% vs physical AoE and 14% vs Magical AoE (maybe even buff the mastery to 5%)