The airplanes seem to be restricted to WAT limit weights even if there are no obstacles ahead...
WAT - maximum Weight for Altitude and Temperature - limit is the Design Standard "bottom line" MTOW for the day and usually cuts you back from the certification MTOW at higher temperatures and altitudes.
Reason for WAT limits is to ensure that you have a modest OEI climb capability regardless of other considerations and checks that the minimum certification OEI climb gradients are available in each segment.
As OS suggests, for most operations you will be limited further by the other limits.