I stumbled over this sentence in a training material about fuel leak:

"If the fuel quantity symmetrically decrease in both wing tanks and the fuel quantity in the center tank decrease, the fuel leak comes from the center tank or the APU feed line."

Regarding the APU feedline part, what I don't understand is if the APU is feeding from the left tank always regardless if it's by the fuel pumps or it's own pump (in case the fuel pumps are off) how can the fuel decrease evenly from both wing tanks?