I think it has a leaf shutter, and a real iris in front of the shutter. This behaviour is quite common, when the diaphram blades are full open the leaf shutter takes more time to open and close since it has to reach the fully open state, whilst when the blades are somewhat closed it can stop before thus allowing faster shutter speeds.
Iirc i have an old Canon Powershot G5 that has the same limits.
The good thing of the leaf shutter is that it allows flash sync at all speeds without tricks high speed sync and so on.
When you need unavaible shutter speeds on the G5 you can use the built in nd filter, even it it's not enough for every situation.