sounds like you're measuring right. Everybody's got different bones, don't let that throw you off. The issue is finding what fits, and there's more to that than sit-bone width. (I think you're lucky, speaking as someone on the other end of the butt spectrum. I have a hard time just finding saddles wide enough!)

So since width isn't your issue, maybe the shape of the saddle is part of the problem? Pear vs "T"? (viewed from above)

I chafe horribly on pear-shaped saddles. My "T" shaped saddle is a dream. The shape you need depends on how your muscles arrange themselves and the angle your thigh bones make relative to the hip socket.

Maybe drag out the saddles you have and compare their shapes? Sort them by which were better and which were worse and see if there are any similarities?