Is "high" on a slow-cooker supposed to mean 300 F?
When I started researching sous vide cooking some years ago, I created a little database of slow cookers on the market and researched the temperatures of different devices. Interestingly, slow cookers are wildly different: low temperatures ranged from 190F to 275F and high temperatures were correspondingly wacky. There is no "industry standard" and depends entirely upon what the manufacturer thinks is a good idea.

One issue you may find interesting is what the real difference between "low" and "high" switch is: it governs the speed with which the cooker gets up to temperature (whatever the manufacturer thinks is a good, poorly-controlled temperature). If you're planning dinner in 5 hours then "high" is the appropriate switch, whereas if you're planning dinner in 9 hours then "low" is the appropriate switch. In both cases, the cooker will get up to the same temperature.

Note that I have been speaking about both the "low" and "high" switch on the slow cooker AND the range (low and high) of temperature which the cooker produces.