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
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.