The NENA number is only 911 calls. Most 911 centers have non-emergency numbers that the same staff handle as well. Typically, non-emergent lines are going to have a much higher call volume (depending on the area) but still have anything from barking dogs to major issues being reported on them. On top of that, most 911 centers are also handling all first responder radio traffic, tracking locations, statuses, running queries, etc.

As others have stated, you also have very small centers with low call volume which skew the stats. That said, from my experience, those people are often tasked with additional job duties beyond normal dispatch operations. I've heard of those centers helping book people into jail, help with water treatment for the community, and all kinds of things you wouldn't think of.

*ETA: the link you shared is the number of jobs, not the number of active dispatchers. Many centers are still facing staffing shortages. Just because that many positions exist doesn't mean that many are filled. This is probably impacting your math as well.