If you exported material from Word, you'll have spans everywhere that have nothing whatsoever to do with Sigil. Word outputs--as nicely as I can say it--gobbledygook--and Sigil attempts to rectify it. If you have something--ANYTHING--in a para that is odd, Sigil will attempt to span it, for lack of any other instruction.
Just cleaning your html in a plain html editor will eliminate a lot of that.
If you exported your html from PDF, even with Acrobat, gods help you. You're better off, IME, exporting to XML and working from there.