Neither Ironforge.pro nor Warcraftlogs are in any way accurate representations of populations on servers. Believe it or not, not everyone who plays on a server raids or competes in Arena combat. Even for raiding, not everyone posts logs to Warcraftlogs. There are many, many people who play the game for fun and not necessarily for posting their statistics online. 
I know there aren’t many alternatives to try to get any sort of accurate server populations, but the numbers from both of those web sites are always going to fall FAR short of any kind of accurate census.
Thanks.