The total length for the scale of the moving part is 19mm.
And it was divided by 10 divisions, so each division is 1.9mm which is less than two units in the main scale by 2.0-1.9mm=0.1mm.
Think of the above system in the following way:
Assume every 2.0 mm in the main part is replaced by a 2.0mm block.
And every 1 division in the moving part is replace by a 1.9mm block.
If the first block of both part are aligned at the right, then there is a 0.1 mm displacement at the left.
If the second block of both part are aligned at the right, then there is a 0.2mm displacement at the left.
I hope you can understand what I mean.
Anyway, I am going to create another java applet to show it.
Drag the slider to change the alignment points. Assume the top block is 2.0mm each, and the lower one is 1.9mm each (total length =1.9mm*10=19mm)