BUT.
Yes, a big but.
Just b/c she measures 40" at home doesn't mean she'll hit their measuring sticks the same way each time. For instance, in September, the *second* CM at Star Tours (inside the queue...the CM outside said he was tall enough) went all weird on DS, made him measure again, and when she could slip a sheet of paper between his head and the stick, she wouldn't let him ride.
A ride in DCA, Jumpin' Jellyfish, is notorious for its measuring stick being "off", meaning a 40" kid might not come near it. But the stick is the last word, no arguing. It's not fair, but they don't seem to be changing (and sometimes a shorter child measures tall enough! I think it's a vortex, ha ha).
A child might measure tall enough 5 times on some ride, then the 6th ride not be allowed to go.
Since I'm a (retired) chiropractor, I have my theories about spinal compression over the course of the day being a part of this, along with being on bumpy rides making us a bit shorter after said rides than just sitting in a doctor's office at home, but it doesn't really matter.
My 3 year old is now solidly over 40", but I still say "let's go see IF we can go on this ride", even if we've been on it successfully before.