Following up on @tiffle's example:
%TriggerVaue% is a token, which means it's only valid in fields that take strings. When you use "Set Variable to Calculation", the "to" field is not for strings, only numbers and functions.
Yes, variables technically only hold string values. But when you use a variable in a numeric field, KM assumes it contains a number, and in fact, the operation will fail if it's not a number.
Also, when you use a variable in a numeric field, you don't include the percent signs or "%Variable%. Again, confusing, but there's a kind of logic there, too.
It took me forever to understand all of this, and I still get confused sometimes. There are visual cues that a field is numeric - for example, there isn't a drop-down allowing you to insert tokens.