Why can the same earthquake have 2 different ratings on the Richer scale and the moment magnitude scale?

This is because some Richer scales are programed or made differently in the factory therefore effecting ones ability to scale an Earth Quake

The Richter scale tells the amount of force that is needed to create the waves and that is used in the moment magnitude scale.

It's kind of like measuring the weight of an object to find the density, it isn't all the information you need.