Thanks everyone!
Interesting input.
I kinda forgot about this because I figured it out. Pretty obvious what it was for when I actually LOOKED at the file.
What remains strange to me is that it only put it in in a handful of places, when there were lots of italics throughout the (very long) book. Calibre, which created the epub file, had translated all the <i> code to a css code and that was present in all the instances where I used ital, including the handful that sigil inserted it's ital into. I'm sure there's a pattern, but I'm not going to bother to find it. I just took out the redundant code.
Thanks again.