Its a new method GBrice ets the credit for. If u wanna look into a recent thread where we go thru it look up:

Trining days vs rest days (i think its called)

The old tdee is inaccurate and too high which is why the deficit had to be 900cals below tdee. U could also use (bmr x 1.26) instead of 1.55 to get pretty close but lbm x 15 is easier to explain