How does this slight misalignment arise? What does one do to correct it? This occurs on every left-right spread,
I only have the same font size page number in the headers.
Have you edited the Default master page such that the left-hand page’s music frame’s top is slightly lower than the right-hand page?
@Lillie_Harris You go it. Thanks!
I think you answered my question even as I hit return, due to a hitherto unknown relativistic time effect.
Fabulous 
for your own information, if that wasn’t the solution - the next thing to check would be in Engrave mode, select Staff Spacing in the toolbox on the left and check for any red staff/spacing handles. If they’re red, they’ve been locally offset. However, when you said this was happening consistently on facing pages, that was the clue it was based on the master page (master pages comprise page pairs: left for even-numbered pages, right for odd-numbered). Anything happening consistently on multiple pages is likely to be linked to the master page.