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.
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe