Looking on the picture, you may be measuring at a slight angle, although it is hard to see.
Yeah, it does look like that in the "staged" photo. I was pretty careful and did the measurement multiple times across several parts, and also across different pin counts.
I couldn't be bothered to try find a micrometer for an extra digit (same as thm_w) as the flex on the pins and general tolerances really don't warrant it, but I did go down to the workshop to get my better calipers.
Anyway, it is fair to say the nominal pitch is 2.5mm (as per the JST XH datasheet) and definitely not 2.54mm. The datasheet explicitly says "Tolerances are non-cumulative +/-0.05mm for all centers". My result is within that tolerance... I was just kinda hoping for it to be exact