Aren't the sounds the same for all 3 units? Thus the 61 demo is no different than the 73 or the 88?

Same sounds - different keybed which can make a difference to the overall feedback loop while playing. The same samples will sound different even if you play using some other controller. Also - the 88 has triple sensor that can enhance how the (same) samples are controlled.

Still, I think the focus is probably more on form factor than trying to differentiate on the sound.