Apple Watch Inconsistent Resting Energy

I use my Apple Watch to inform a calorie counting app to help with weight maintenance. Recently my watch has started reporting wildly different resting energy totals from day to day (one day will be 954, another 1200, another a more close to accurate 1400). From what I understand about how Apple calculates the resting energy portion of total calorie expenditures— there shouldn’t be variation of this nature in resting energy. Any thoughts on why or how to fix? Throwing my diet app way off.