Version 4.2.1 map distortion?

Now that you mention it, I have had the same feeling.

That isn't new in 4.2.1, in fact this can been seen in every version of BaseCamp. It depends on the area the map spans; that differs between the 2010 and the 2013 map.
It can be solved by setting the projection angle. To do that you need to hack the registry (certainly not the preferred way), to use the undocumented key combination Ctrl-Alt-A in MapSource or to use JaVaWa GMTK. The latter works even on the Mac.

Note: this works only with maps that are installed on your computer. If the map is read from the device by BaseCamp you can't use MapSource or GMTK to set the projection angle. A solution is on the way...

Now you can accomplish the same thing for maps that BaseCamp reads from the device with the latest version of JaVaWa Device Manager