The very exhaustive documentation above may help. I just want to add that you can "buy" different MTOWs from Airbus. Normally, the heavier the more you pay. You can even change your MTOW later (by paying more). Technically, there is no difference.
Most A320s in Europe have the 73.5 you mentioned above. But out of Europe, mostly in Asia with their longer flights, they rather choose the 77t version. It's all a matter of your network. In Europe you need less trip fuel so you rather stop at the MLW or MZFW.
hth,
Dani