Traditionally, with the exception of a full court wedding ring, wedding rings always had a straight fit on the inside. Recent times have seen a shift towards designers using the softer feeling curved interior known as the 'Comfort Fit'
Comfort Fit rings are easier to take on and off due to the fact that there are no sharp corners to dig into the flesh of the finger and get a grip. People with larger knuckles will see the advantage of this as the fit allows the ring to slide over much easier.
Simply put though the reason they are called comfort fit rings is because they are more comfortable to wear on a 'day in, day out' basis than old fashioned straight bore rings.