The size of the bar at the stem clamp is irrelevant. Whether the bar is 25.4, 26.0, 26.4 or 31.8, it'll still be the same diameter at the grips.

The catch is that bar-end shifters are generally used on road bikes, whose curved bars are typically 23.8 mm outside the stem clamp area. Flat bars are typically 22.2 mm. The narrower outer diameter generally means a narrower inside diameter, and in this case it was too narrow.

Steel bars normally have narrower walls than aluminum ones, so it makes sense that your shifters fit into your Nitto steel handlebar.