The inner layers have a different voltage spacing rule than the outer layers, much less spacing required. The top and bottom pads maybe non existent and the holes plated through.
This is quite true, and it is a very curious thing.
Two electrodes in air have a much higher breakdown voltage than if a flat surface is introduced over which an arc can travel. Even very good insulators like fibreglass, mylar, or real glass cause this increased surface arcing phenomena.
But a very thin sheet of the same material placed at right angles blocking the path between the same two electrodes, hugely increases the breakdown voltage.
Inner PCB layers increase the voltage rating between pads, and so will milled slots which break this troublesome surface arcing effect.
And surprisingly so will an ordinary solder mask.
For high voltages what you can do, is use relatively large area pads which may be required with very wide tracks to carry high current.
Then edit the solder mask layer for a smaller pad size than the real pad underneath.
Just enough exposed copper area for a neat tight solder fillet, and no more, and cover up the outer edges of the pad with solder mask.
This is quite effective, I have tried it and it works.