Thanks for the comment. As always the actual altitude is up for debate on most mountains. The research, via GPS, done by this professor http://www.utsa.edu/pub/sombrilla/pdfs/summer2003.pdf (pg22)is the most exact that I have seen. My gps was showing around 18500 when I climbed it for a non scientific test. For now I'll take the professor's research over a guides. Cheers!
I agree with you, you could go back tomorrow and get a whole different reading. My altimeter showed 18166 at the crater but I wouldn't rely on it as temperature greatly influence any altimeter's reading. So, we can happily say that Orizaba's altitude is somewhere in between 18,000 and 19,000 and anyone wishing to know the EXACT altitude, can summit and get his own reading!!!
No matter where you are, guides will probably usually use the highest elevation available.
Anyway, it is 100% certain that Orizaba is around 18,400-18,500 feet rather than 18,700 feet as some older sources state. SRTM data confirms this and 18,700 is for sure an error. The latest Mexican topographic survey puts the peak at 18,409, close to the height listed on this mountain page. SRTM, topographic surveys, and the figure posted on the mountain page are all fairly close.