Most of my knowledge here is coming from working on my 1970 Chevy Pickup, so some of it may not be exactly accurate for your vehicle...
There are basically 3 things you need to consider for timing - engine RPM, air/fuel ratio, and volume of air fuel mixture in the cylinder.
Engine RPM is simple, as the engine spins faster, you need to ignite faster to compensate.
A/F ratio - Richer needs LESS advance. More fuel burns faster.
Volume of A/F mixture - more volume = less advance. If you have more air and fuel, it will burn much faster. This is what vacuum advance takes care of on older vehicles. Think of the amount of air/fuel in your cylinder at idle versus the amount at WOT.
On the small block chevy's (and I believe most carb engines) you typically set the timing for all in at ~36*. You hit that 36* early, like 2200 RPM and it will stay there through the rest of the powerband. Vacuum advance adds ~10-12* of advance at idle and 0* at WOT.
By the way, I can't see your image, so I don't know the range we are working in here. This is not your fault, I'm at work...
What might be happening, likely a combination -
ECU is richening the mixture to help counter the heat in the cylinder (common at WOT)
ECU is pulling timing to be safe at higher RPM.
If you were not wide open, then went wide open, the timing should retard quickly.
I believe volumetric efficiency increases with RPM (especially on a DOHC engine)