That was done because the labels are shown in different contexts. When there is a very small amount of space to show information, I use 2 if it is ST2 only, 3 if it is ST3 only, or nothing if it is compatible with both. However, when there is more room in the design, I use ST2, ST3 and ST2/3.

Additionally, I made pretty liberal use of the title attribute on the site. Most things that are not obvious can be hovered for more information.