Why footprint dimensions are to edges and not centers
I have drawn many footprints/landpatterns i different ECAD tools. 

How the recommended footprint dimensions are specified differs a lot between manufactures. Some have dimensions that can be used right away, others you have to calculate the dimensions you need to draw the footprint.

My guess would be, that some of the guys putting on the dimensions in the datasheet don't know how they are used by the PCB designer/layouter.   

It's mostly a "problem" with special components that don't use standard footprints like connectors and modules.

Most ICs and passive components have the dimensions on the components specified in a similar way. Which makes it easy to apply the dimensions in a tool like PCB Library Expert, which will generate a footprint according to IPC recommendations based on the dimensions of the component.