I also write left handed and mouse right handed. My theory is that it’s more efficient when using a QWERTY keyboard layout. The QWERTY layout is heavily left dominant in English and, I guess, many other Western European languages (https://en.wikipedia.org/wiki/QWERTY#Properties: “In the QWERTY layout many more words can be spelled using only the left hand than the right hand. In fact, thousands of English words can be spelled using only the left hand, while only a couple of hundred words can be typed using only the right hand (the three most frequent letters in the English language, ETA, are all typed with the left hand)”)

So chances are that the last key hit before using the mouse is on the left side of the keyboard, as is the first key you’ll hit after using the mouse.

That makes mousing with the right hand while keeping the left hand on the home row faster than the reverse.