I had it happen with one of my foster babies and it was absolutely just a matter of a different person doing it differently with a squiggly baby. I guess they all have a "method"... kwim? In my case, the child just hadn't grown and measured the same as at the prior appointment (which they didn't LIKE, but she was apparently on the verge of a spurt--so at the next appointment she showed growth).
Different people do it differently, sometimes they measure at an angle and get an exaggerated result. I think you would be able to tell if all of a sudden clothes fit weird. It's all about the experimental error of taking the measurement. I wouldn't worry.
IMHO, I would not worry about this at all!!! I'm kinda surprised they are having you come back in. MY son is always all over the place and has been like 2 inches shorter from visit to visit, clearly human error, not that my son is shrinking. I would trust your intuition, if everything seems to be going well, eating, sleeping, etc I wouldn't stress about the Dr's measuring error.
This wouldn't bother me at all and I wouldn't be making a separate trip for another measurement unless there were other concerns, like baby is not eating, stooling or voiding, etc.
Also it could be that he is the same length but went up in weight. My babies always alternate between weight and height growth spurts. Their weight will not go up much but their height will and vice verse.
Don't worry mama I am sure he is just fine.