Jubilee and Met trains use the same tracks in and out of Neasden Depot. So it's not practicable to keep their electrical supplies entirely segregated. Therefore Finchley Road to Harrow, which is all one electrical section, stays at nominal 630V supply but S stock regenerates to 790V in that stretch and has done since about 2014 if I recall correctly. 1996TS on the Jubilee also does 790V regenerative braking, but isn't rated for 890V.
Therefore Finchley Road to Harrow, which is all one electrical section,
I thought there was a sub gap on the approach to Preston Road?
There is a sub-station gap at Preston Road Sub-Station, but it's not a Sectionalisation gap. Not all sub-stations are at Sectionalisation boundaries. Indeed there are other sub-stations between Harrow and Finchley Road.