That means that haste is stronger with your current stats. Since haste and mastery scale off each other every point of haste will make every point of mastery stronger, which means that if you keep stacking haste mastery will be stronger at some point (in most cases around the amount where you get more haste than mastery, hence the 'general rule').
However, basically haste/mastery complement each other. Running sims only gives you weights on your current stat setup. If it says haste is way above mastery, reforging out of of mastery into haste isn't necessarily going to boost your DPS. If you did so, and ran sims again, the weights would be much closer or perhaps even now favor of mastery! Use reforge plots/scaling and keep at it until you are at the optimal point.