As noted, Keyboard Maestro uses double to store values internally, and double has about 15 digits of precision, as well as using 64-bit for integer calculations (about 19 digits total).
So if you try to do anything that expects more than 15 digits of precision, or in cases where integers are involved, more than 19 digits total, you are going to be disappointed and/or surprised at the results.
So in the case of 12345678900000000000, that exceeds the size of a 64 bit integer and in this case you are doing integer maths, so the result is an overflow.
Anything that exceeds precision or digits will result in potentially incorrect numbers, and this is not treated as an error (it basically never is in computer speak).
Try it in Numbers for example, make a cell =12345678912345678912.