We introduced a bug when we fixed something else in the UI. So entering a negative number is what is broken. Beyond that, the arithmetic for both works as expected, including decimal values. It also appears that you can't enter a decimal point.
Same results on current build, except now that I can enter a negative value I get an appended -10 even instead of math. This is using a "number' based GV with a "number" connector. Same test with GV without a connector works as expected (math works). Issue seems to be caused by adding the connector to the GV, even after I remove the connector the math on the variable value no longer works (just appends the value).
FYI It does the same thing with a decimal variable connector, I'm not sure if a dimmer or volume would work for my application, aren't they limited to a range of 0 to 100?