This is a function of the speaker change in reactance as the cone is driven. Very little in the cabinet, but it does impact the speaker a tiny bit. Small changes in the cone position (low volume) won’t change the load on the amp much. Loud volumes will present a lot of change to the load of the speaker. Think of how an open back cabinet will be a little different than closed or ported. Not only is the sound directed differently in each case, but the air resistance changes how much the speaker needs to push against the air and how far, and then how far into the cone’s suspension (foam stuff around the perimeter), and other things like that.
This is one of the interesting things of a tube circuit. Tubes have a high drive impedance, but high voltage, so we use a transformer to get down to the impedance levels needed to move a speaker. Changes in the speaker alter the tube response because the change transfers back across the transformer.