That may be how it goes, but they SHOULD have done it like this.
say in the last 365 days you`ve spent $3000 and on December 28th of 2015 you spent $500.......Tomorrow, your total amount spent will go down to $500 and on the anniversary of a day you spent more $, that amount will drop down by that exact amount. As long as you keep spending through the year, your amount should go up and down depending on that same thing.
It makes sense they`d do it that way now, since its supposed to be going by how much you spend while a member.
I get what you're saying, but I'm sure they rather take the easy route and have it reset on your join date. I just keep track of it myself.