This is not an issue with /var itself. The /var command can perform simple calculations in the same way as /set. Both of these commands are used by many scripts for this purpose. It would not be possible to make changes to how these commands work without affecting all existing scripts.
The /set command has an -n switch that allows you to store input without evaluating it, as you discovered. The -n switch is not available in /var, which only supports a subset of the switches available to /set. I will add this switch to /var in the next version.
