I think AMR's Raid Heal/Max Heal stat weights are more for healing as long as possible without going out of mana. Therefore, their stat weights value Crit moreso than Mastery due to refunding mana. Where in real life, Crit is definitely useful to maintain mana, but Mastery allows for more throughput and overall bigger heals. Once you reach a certain % of Crit (usually 20%-30%), Mastery becomes the stat to stack as high as possible.
You can do custom stat weights in AMR and put a cap on Crit and see how that works out for you. For my normal Raid Heal setup I have about 24-25% crit with about 120% mastery. Ideally, if you struggle with mana might want to bump crit up or use a mana-regen/mana saving trinket. Really depends on the fight.