There are many ways to formulate an epoxy product, emphasizing various properties, depending on the intended application, putting in or leaving out various additives. It would take a polymer chemist to detail the possibilities, and maybe one such will come in and tell us a bit about all this.

'Epoxy', as I understand the terminology, is the cured plastic end result. 'Epoxy resin' is what becomes 'epoxy' after it has been thoroughly mixed and reacts with 'epoxy hardener'. So the resin is not epoxy, strictly speaking, nor is the hardener epoxy, but they combine and react chemically to form epoxy.

When you thin epoxy with alcohol or some other solvent, mix thoroughly, then spread it out in the thinnest layer you can, the alcohol easily evaporates before the epoxy cures, having a large surface from which to escape. When you form alcohol-diluted epoxy into a multi-layered lamination, you have more alcohol concentrated in a more three-dimensional volume, with less surface from which to escape. That leads to the epoxy curing before the alcohol has completely evaporated, giving you a more flexible product. Most likely the alcohol will continue to evaporate over time, perhaps stiffening the piece as it goes. Or maybe not. In any case, my practice is to thin with solvent only when I am putting a glass finish on, and not for structural laminations.