Looking at the image in post #299 (for which many thanks), it appears that the terminating Liverpool platforms curve to the right (North), as if to avoid the arrival and departure siding for the freight depot. Old maps show that it previously had connections both ways and its own sidings, for the ROF and later Kirkby industrial estate.
Also, the "projecting" Wigan platform appears to be precisely in line to serve the down line from Liverpool and so would be in the right place for a future (and ideal) through, 2-platform station. Perhaps this is why it is a permanent 150m long.
The Liverpool terminating platforms seem curved, unusual in a new station, as this can cause gaps. The freight siding looks rather close to, or actually on, the up (Liverpool) track bed, so it may have dictated the shape of the Liverpool platforms, as it is c600m long and might be expensive to move.
Perhaps others could comment further?
WAO