[Brightcolours]

How is it possible to get higher resolution values on sensors with lower pixel densities and AA filters?

It is not possible (unless you mess things up as camera manufacturer with wrong thickness sensor glass or something).

It is the testing methodology's fault, where sharpening is applied to the MTF test shots. The lower res. images extreme corners get lifted up quite significantly, and the higher res. extreme corners without AA-filter do not get touched by the lower amount of sharpening applied. 

 

By the way, a lack of AA filter mostly adds fake sharpness to MTF tests, not real sharpness. But at parts where the optics are not thaaat sharp, the optics act as AA filter and you won't see the MTF results being muddied by the fake sharpness.