Part of the answer is a combination of engine RPM and gearing. Generally, vehicles get the best fuel economy cruising in the highest gear at the lowest possible engine RPM. Brisk acceleration normally demands lower gears are held longer while the engine runs at higher RPM (particularly true if you're driving an automatic).
At higher RPM, your fuel economy display shows you the cost of increased internal engine & transmission friction and accessory drag, to start.
People are going to correctly point out that you
can accelerate with a high engine load and moderate RPM (manual transmissions work best for this) and take advantage of the engine's "performance curve", as you say, to minimize fuel consumption while generating the higher power needed to accelerate ("performance curve" in this respect = Brake Specific Fuel Consumption (BSFC):
Brake Specific Fuel Consumption (BSFC) Maps - EcoModder).
But operating an engine close to its best BSFC point on the "curve" (map) does not equal lowest overall fuel consumption. Some people will tell you that it's
always best to accelerate briskly for best overall fuel economy, but it's not as simple as that, as your dash display shows. A lot depends on "what happens next" (after accelerating) in your particular driving scenario.
Having said all this, I think people worry far, far, far too much about acceleration technique. Much more fuel savings are to be had by most drivers by focusing on the brake pedal (minimizing its use) and by cruising at moderate speed
after accelerating (on the freeway).