An offset press takes a bunch sheets run through before it will print optimally; sometimes the pressure on one or more plates has to be adjusted, sometimes the plates are misaligned, it takes a little bit of running to get the ink flowing properly to the plates, and to the substrate that is being printed upon.
A bunch of sheets will be run through the press (called "make-readies") to get the press warmed up. After a bunch of make-readies are run and any issues addressed, you then take one as a "press sheet" which is used to confirm with the client that the job is acceptable before the order is run.
Once the client signs off on the press sheet, the order is run. As presses run extremely quickly (there were several that did over 10,000 impressions per hour when last I worked in prepress), an order is generally plus or minus 10% for most commercial jobs of any decent size. It's difficult to get an exact number with sheets going through so quickly, with a small percentage of spoilage at the press. There's also a bit that may get spoiled during finishing, so a tolerance is built into an order.
An uncut sheet is what the printer would show to the client for their approval to print the order. I'm guessing that the uncut sheets are the overage, taken by the client before the sheets are die cut into cards. Or maybe they specifically order a small number of press sheets.
In any case, that's what an uncut sheet is for. The press crew gets a press sheet ready, and it is approved for production, or rejected.