It is not an efficiency issue. It has to do with how they list the HP: full rated, max rated, and up rated. The 14 Amp motors are larger motors than the 11 Amp motor, even though they are all called 3/4 HP. Sometimes a 3/4 HP full rated, 1 HP max rated, and 1 1/2 HP up rated motor will all actually be the same motor. The labeling approach varies a little from manufacturer to manufacturer, so the ratios I just gave aren't nessessarily representative.
What you should really be looking at is the brake HP, which is the listed HP times the service factor.
What they don't offer is a dual speed version of the smaller motor. Historically you only got a two speed motor when you were getting a really large motor. The manufacturers have started to shift this down so that smaller motors are now offered in two speed but many of the dealers only carry the larger two speeds. Even so the smallest motors don't yet seem to be available in two speed.