This is a remnant of British colonization. French-speaking population didn't know any English, so you have a lot of these literal translations.

I've heard "flour" uttered with the French pronounciation (fl-oo-r, instead of homonym of "flower") in New-Brunswick. I was floored. Took me a while to figure out what they meant.

Clearly, this originates from non-English speakers reading "flour" on a sign and just running with it.

Also, consider that the British conquest happened before watermelon was highly prevalent in France or North-America. It's unsurprising to see terminology diverge in this case.