The seat map is showing which seats are available for you to select. The inventory is showing how many seats UA is willing to sell. The two are not directly related.

The difference between the seat map and available inventory can be caused by any or all of :
* Passengers that have not yet selected/been assigned a seat (especially things like tour groups where this is common)
* Blocked seats, due to things like pilot rest seats (in particular on some flights the seat next to the pilot rest seat will be blocked until close to takeoff, and only assigned if it's needed - although that wouldn't be relevant on this config)
* Seats blocked for other reasons, such as being held for potentially disabled passengers (normally released at 48 hours, but not always)
* Inventory in J being reserved due to Y being oversold.
* A willingness to oversell J (in general not done on United, but still possible)

Or potentially many other things.

the seat map is a good way to get a general idea of how full a flight is, but it is no way a perfect indicator - especially in Economy (where things like Economy Plus and Basic Economy come into play), but even in Business.