Pretty simple and reproducible bug. Here's my rule and the resulting local vars' values immediately after running its actions. The offset parameter doesn't seem to do anything.
Should have mentioned I'm on HE platform version 220.127.116.11.
I just tried this out and got the same result with a number and decimal variable.
Just tested this and the offset didn't work. I was able to add another rule after, then add number to it, and that worked.
In your case, add a new rule "Add 5 to Var"
(Set Mode or Variable... -> Set Variable -> Var -> add number -> 5)
This was explicitly fixed in a later platform release.