212F (100C) is merely the standard, defining the boiling point of pure H2O at 1 bar pressure. In a way, one definition of pure water is that it boils at 100C at 1 bar.
The technical term for the situation of a liquid boiling by natural convection is "pool boiling." The dynamics are interesting and somewhat involved. The bubbles formed take energy out of the system. And your cooker generally has a mixture of substances, all of which have different boiling points. Very pure substances may do nothing until the liquid reached boiling point, and then they can suddenly boil explosively. Even tap water is not purely H2O.
Plus, there may be variations in the boiling point from the standard at every elevation except the ideal 1 bar pressure at sea level, the boiling point being lower at altitude and ever so slightly elevated at some place like the Dead Sea, or a lot higher in you pressure cooker. You get your tea a tiny bit quicker (and cooler) in a hurricane. And there are temperature gradients in the liquid (and those difference change with temperature) and differences in temperature at different points on the bottom surface of the pot.
And the temperature controls on a slow cooker are not very precise, whether there's a thermostat or not. The exact temperature of the simplest cooker varies somewhat throughout the day, as line voltage varies slightly. And the maker had to choose some point at which to measure temperature and had to decide how much difference to allow between the sensor and the interior of the pot. Since it rarely matters whether actual bubbles form, so long as the temperature is reasonably close to the target, the effect on the food is the same within a range of close temperatures.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen