Boeing 737-900 HGW with max of 189 seats

On the new game world Ellinikon, I operate 5 Boeing 737-900 HGW which are about 13 years old. The configuration is Y174 (Lesuire) C 15 (Recliner Long Haul). 189 seats which is my seating configuration is also the maximum which interests me. I would think the aircraft would have 200 seats in a standard 1 class configuration but I checked it out and only 189 (same with slimline HD) with a lot of space left over. 

Why is there a maximum seating capacity of 189 seats when there is space to use. However I know beoing have this as their maximum as well. 

has to do with maximum take off weight. the HGW (high gross weight) version has the same hull as the BGW (basic gross weight) but can carry upt to 220 pax.

Edit: assumed he was referring to the ER versions.

The 737-900 has a limit of 189 due to the number of emergency exits. The 737-900ER has extra exits so can accept more seats...