Why do we use a single 'dash' for difference : $-$ and a double 'dash' for sum: $+$?
Just a shower thought: Who came up with this notation? It kind of makes it look like the difference is simpler than the sum, when intuitively, in my opinion, it should be the other way around.
This set of web pages is a good quick source of information on such topics, and I can add little to what is found there. It would not at all surprise me, however, if the abbreviation for Latin et mentioned there was a version of the very widespread Tironian note used to abbreviate et throughout the medieval period and later. Some versions looked rather like this $7$, but others looked like a crossed $7$ and hence a bit like a plus sign. Here’s a picture of one such, taken from Wikipedia:
You can see how it might easily turn into $+$ if written hastily.