There is a difference between the Power transfer unit and he Landing gear transfer unit, they are different systems.
FCOM Vol2 13.20.3
"With fluid level at the top of standpipe, fluid remanining in the system B reservoir is sufficient for power transfer unit operation"
FROM HERE I GUESS THAT PTU USES FLUID REMAINING FROM THE SYSTEM B RESERVOIR.
"The purpose of the PTU is supply the additional volume of hydraulic fluid needed to operate the auto slats and leading edge flaps and slats at the normal rate... The PTU uses system A pressure to power a hydraulic motor-driven pump which pressurizes system B hydraulic fluid"
FROM HERE I GUESS THAT SYSTEM A FLUID MOVED THE PTU MOTOR DRIVEN PUMP TO COMPRESS HYDRAULIC SYSTEM B FLUID REMAINIG.
So, there is no change of fluid between system A and system B.
