I am not at all certain why they droped the "e" from "Tussey". However, this is certainly not the first surname line that I have come across that letters have been droped, (i.e., Robinette = Robinett =Robnett, Prater = Prather, etc.)
Generally, I find that it traces back to an ancestor that could not write his own name and someone spelled/wrote his name for him. That person spelled it as it sounded.
In a few other cases, it appears that a child wished to disassociate himself from part of the family for some reason or other and changed the spelling.