Well, you roll the ball at a set angle from a set position, thus if properly measured it will exit the stimpmeter at the same speed every time. The reading you get is how many feet it takes friction to stop the ball. You are really measuring rolling friction, not speed (sorta like "rolling resistance" in bicycle or automobile tires)
When TEPaul uses the word "exponential" when talking about greens stimping over 11, he's talking about what you see on a green with a fair amount of slope. Because the greater the slope the lesser the friction, it takes less slope on a green with less friction (higher stimp reading) to get to the point where gravity overwhelms friction and the ball won't stop until it reaches an area of lower slope or higher friction (i.e., off the green surface)
At 10 perhaps only 1% of a certain green's surface renders the ball nearly or completely unstoppable. At 11, maybe its 10%, at 12 maybe its 90% (not unreasonable for a green that's a large overall tilt, like most of those at my home course) Hence, exponential.
I don't know what polished concrete or marble floors stimp at, but I would imagine it is probably 20+. But anyone could putt those "greens" without any trouble once they hit a few putts, because they are flat....well, maybe not the floors in the Leaning Tower of Pisa
If you had a polished concrete driveway that's got the slightest amount of slope (probably 1% grade would be sufficient) it would be unputtable because once a ball starting rolling on it, gravity would take over and it'd end up in the street. Note that I'm talking about polished concrete, even smoother than than the smooth concrete you might have in your garage, definitely not the deliberately roughened surface you see in most driveways, sidewalks and streets!