What went wrong with my bottom bracket spindle length calculation?
I went to take pictures of the BB installation as @Batman suggested. After removing the crank arm, I actually noticed an offset of 1-2 mm on the drive side, like @mattnz anticipated. However, I also noticed that there was a gap of about 1 mm between the frame and the BB lid:



After this observation I googled a little bit, and learned from this forum thread that the drive side main body of the BB should be installed before the non-drive side cup. I had installed the parts in the wrong order. I loosened the non-drive side cup a little bit, I was able to screw the main body in a couple of turns more. This eventually resulted as a chainline of 50 mm, which I find totally acceptable, as my target was 49 mm. The non-drive side cup sticks now out about a 1 mm or so, but the above mentioned forum thread suggested that this is OK as well.

After the corrected installation, I can assure (for future reference), that at least this my UN55 70x122 bottom bracket does not have any offset at all; length from end of the spindle to the center of the frame is equal on both sides.

I think that I've found answer to the math side of my question as well:

By combining info from pages 16 and 34 of this PDF I was able to find out that 122 mm Shimano BBs should give a relative chainline of +8 mm compared to the 110 mm ones. So if 110 mm BB should give a chainline of 42 mm with my crankset (as was the assumption), 122 mm should give 42 mm + 8 mm = 50 mm which is exactly what I'm seeing now.