The OUTER wing tanks on A320 only hold 690kg/1500lb, and was designed into the system to reduce wing tip flutter at some high Mach Nos. & FL's.
It remains there in the cruise until the INNER tank capacity reaches 750kg/1650lb, the transfer valves then open allowing this OUTER fuel into the INNER tanks, and becomes available for use.
This usually occurs just as the aircraft pitches nose down to start the descent, but obviously depends upon initial fuel load.
Note, the A321 does not require this feature, and only has a single tank in each wing, very simple.
Roger Miller.