Suppose I have a configured tab width of 2 and I write a line with three tabs that's 78 characters wide. When my friend with tab-width 8 views it, he'll see a line that's 90 characters wide. What do projects with 80 character limits do in situations like that?
In either case the line contains only 75 characters, of course, but that's a pretty pedantic non-answer: we care about horizontal screen space, not the number of bytes of storage you'd need.