When you get an unexpected result from SimulationCraft, you may have reached or passed a stat cap or plateau that can cause the value of a stat to change significantly. Under the Options tab, go to Reforge Plots and set a reforge amount of 1k or 2k, with steps of 100, 200, or 250. Make sure to check "Plot Reforge Options for" both Haste and Mastery. Enhancement is a weird spec, and stats bounce around in value all over the place. On your Results page, you'll end up with a chart like below (I ran it with reforge amount of 1000, steps of 100):
I've labeled three points on this graph:
Point
B is your DPS as it stands. With my Haste and Mastery sitting where they are, I'm projected to do 207,512 DPS.
Point
A is what happens to my DPS if I reforge 100 Haste to Mastery. I should lose around 150 DPS.
Point
C is what happens to my DPS if I reforge 300 Mastery to Haste. I should gain around 50 DPS.
Those DPS changes are pretty close to the error margin, so it's not worth making any changes to my build based on that data. If the difference in DPS were more significant, that might help me identify a breakpoint I want to reach for Haste before dumping more points into Mastery. Or vice versa.