I was thinking about this today, reading about people's whose floors buckled, swelled, gapped etc. I've read that it's normal on some pre-finished floors to get some slight gaps in the winter when it's really dry out, and that they will swell back shut when the humidity rises (correct me if I am wrong).

How does that work with a site finished floor, where the entire surface is then one solid sheet?

Wood is wood. The finish is not a vapor or moisture barrier.

I see. I was under the assumption that with a site finished floor, that top glossy coat hardens into one solid "sheet" if you will. Is that not the case?

If the floor boards shrink and gapping occurs, then the finish usually cracks along the seams of the boards. Sometimes, certain finishes will stretch over a smaller gap.

OK that leads me to another question then...is gapping normal in the winter months when it is very dry?

Basically, yes. But it can be prevented or minimized by adding humidity to the environment. By using an inexpensive humidistat, one can monitor their relative interior humidity and add humidity as needed. Simple measures can be boiling pots of water on a stove top or simple humidifiers. The reason wood shrinks horizontally (across the grain) is due to loss of moisture content. Boards do not shrink lengthwise; only widthwise. This is due to their cellular structure and the way boards are cut. Also, by using quarter sawn flooring, expansion and contraction can be minimized. Plain sawn solid flooring is more apt to expand and contract than engineered or quarter sawn flooring. Prefinished flooring, with it's eased/micro bevels, expends and contracts the same but it is typically less noticeable due to the eased edges.