Notice that the smallest side (1) is opposite the smallest angle (30°), and the longest side (2) is opposite the largest angle (90°).
So while writing the ratio as 1: √3 :2 would be more correct,
many find the sequence 1: 2: √3 easier to remember,
especially when it is spoken. See also
Side /angle relationships of a triangle.
In the figure above, as you drag the vertices
of the triangle to resize it, the angles remain fixed and the sides remain in this ratio.
If any triangle has its sides in the ration 1 - 2 - √3, then it is a 30-60-90 triangle.