In order of disparity:
1. Differences between manufacturers
2. Differences on standardized rims used for the nominal measurements
3. Variance in product of the same manufacturer.

I think it's very unlikely that you'd see a substantially different width in a road bike tire between 90 and 120 psi on the same rim; but I haven't checked.

There used to be a story that one manufacturer would make (eg) a 22mm tire and label it as a 23mm tire so that when you compared weights among 23mm tires theirs would be lighter. You don't hear that so much anymore.

And yeah, my bike that can take Vittoria 27s with plenty of room won't fit a Conti 28 at all... I don't know if that's a consistent difference between the two brands, but it's enough to make a different decision next time.