Hot to ground is 120v but hot to neutral is 90v
Basically the neutral has current trying to return back the the transformer and is dropping voltage due to how a neutral is simply a series circuit.

When the source hot passes thru the light (as a parallel circuit), it continues out the neutral without dropping voltage.

The problem with this circuit is the neutral and hot do not originate in the same junction box and is very confusing not only DIY's but also electricians, not to mention a Code violation.