I thought that the voltage between two power sources connected like that would be the higher voltage.
Naw-uh. Why on earth would you ever expect that? Associated with this, if you
connect a 9V battery to the 120VAC mains, the battery won't read 120VAC, it
will explode.
Actually what happens is that the voltage at the middle node will be the weighted
sum of the 2 voltages, as weighted by the output impedances [ie, resistances] of
the two sources.
V = Vlo + (Rlo / (Rlo + Rhi)) * (Vhi - Vlo)
In this case, Rlo << Rhi, so V = Vlo approximately.