In reply to Vertical Dimensions of Adobe's Source Sans Pro:
Alignment is not as important as apparent size.
Relative vertical proportions of ascender, x-height and descender are also relevant, which may explain the variation in cap height.
If you think there is a problem with the dimensions mentioned here, it would be best to illustrate it with a specimen of “real” running text, rather than in the abstract.
Many well-established faces have x-heights which vary according to weight.
Gill Sans, for instance.
It’s a practice which I’ve implemented in many of my designs.