Phoinix; We had exactly the same; 159 till 139 after XFER pumps OFF, sucking at it for almost 15' before supply tank indication dropped."
So, a 20kg decrease in that time....I observed the exact same behaveour in two out of two checks I did in our company. Only difference: we started at appr. 240kg in the main tank decreasing to appr 220 before the supply tank quantities started to decrease.(in about the same amount of time)