Probably because your recording format setting was at 60fps. YOu have to set the recording format to 720/24PN (menu #5) and then in the menu #1, line 2, set it to 60fps and you will get 2.5X slo mo.
I think when you slow down in post it duplicates frames or interlaces them. If you shoot at say, 48 frames per second then you have 48 full frames. Much better.