>>51063009TL;DR
writing with your left hand means you brush over the newly written text to write the next word (assuming you write from left to right).
Since fountain pen ink can take a while to dry (depending on the brand), the paper can become smudge-fest central.
So to minimize smudges, it's usually recommended for lefties to use a quick drying ink in a fine nibbed pen (which minimizes the amount of ink on the paper = faster drying = less chance of smudges)
