Cable head is too small
Sounds to me like you've got Campagnolo-sized shifter ends.  The difference is subtle but definite.
Left is Shimano, Right is Campagnolo.
Not sure how you're measuring, using a ruler might explain the difference. The common-quoted diameters are
Shimano shifter end barrel diameter: 0.173&quot; (4.39 mm)
Campag shifter end barrel diameter:  0.159&quot; (4.04 mm)

Generally speaking, campagnolo stuff is more expensive because it is less common, because it is more expensive (loop!)  These shouldn't cost more than a couple of dollars each.