>linear correlation means that the degree difference should be the same, like if the difference between -5 and 5 is ten then the difference in Fahrenheit should also be 10, but it's not, which confuses me severely

Linear correlation means "if you change the °C by 10, then no matter what the original °C was, the °F changes by the same amount (not necessarily 10) each time".

Or, more simply, if you plotted °F against °C, the graph would be a straight line.

The slope of that line doesn't have to be 1.