It's due to the unusual way they implemented it. Watch gigacontrol when you use tx slowing and you will see that it is the horizontal marker that moves slowly, so the servo which is represented by the vertical marker, slowly follows any programmed curve. But if you use normal tx speed and gigacontrol slowing, it slows the vertical servo marker not the horizontal marker so it can not follow a curve. Wea then moves the servo in a straight line from the start point on the vertical axis to the end point on the vertical axis at whatever slow speed you have selected. If you have a top hat shape curve, the start and end points are the same on the vertical axis so there is no movement of the servo at all. What you had done by having both tx and Wea slowing was mixing the two so the tx control moved to the middle of the top hat and instead of the servo going straight there, Wea was slowing it down in gettting there, so the servo just started to move by which time the tx control signal had reached the end of the top hat thus commanding the servo to come back to where it started. The result was that you got a slow small servo twitch. By switching off wea slowing you allow the servo to go at full speed to follow the curve at the speed set by the tx.