I was just on the point of writing some QA tests to ensure that glyphs' points which should be on the baseline/xheight/capheight were actually at the appropriate Y coordinates - allowing for overshoots, but flagging points which fall one or two units either side of the metric as a potential mistake.
To test my tests, I reached for a copy of Source Sans Pro in order to modify it. But I found that the Y-coordinates of the cap points in Source Sans weren't at the OS/2 sCapHeight metric anyway: sCapHeight is defined as 660, but the top of the capital A, B, D, E, etc. were all aligned to Y=656.
Is this common? Is it a mistake? Why would you define a cap height metric and then not put your caps on the cap height?