1992 MBK Mirage Pro - Colombus Acciaio Aelle Ellipse Tubing - Suntour SL to 105

I had the same issue setting up Shimano 10sp double but got there eventually after just patiently trying slightly different tensions. That 3rd movement (not a click though on my Ultegra) gives a bit of trim for when in the higher gears.

This. I think that trim feature is common to shimano shifters up to 105 (at least the new internal 105). I may be wrong though.

It often confuses people. Used to get loads of queries or returning bikes with that issue when I worked in cyclescurgery. Used to be so frustrating to see bikes booked into the workshop, and mechanics fruitlessly working on it, because the sales guy hadn't actually paid attention to the problem.