During testing in Dashboard, I clicked my "Number" variable's tile, and changed its value from the default of 1 to 5.1 and clicked SAVE. The tile now displays "5.1".
Back in Settings ► Hub Variables, I notice that the corresponding variable now displays null as its value. Returning to the Dashboard, the tile display is blank.
Seems wrong on several fronts. Dashboard could benefit from data integrity checks, or red-text warnings in the DB UI, or automatic truncation of decimals, etc. NOTE: Hub Variables already will not permit a decimal point (.) to be entered for "Number" vars.
I also vote that instead of "Number" for this variable type, we use "Integer" instead, to further distinguish it from "Decimal". (After all, Number implies literally any number.)
