I believe you've hit the nail on the thumb right here.

And the only thing this script is doing differently than my actual script is reading the values from Excel.

Depending on how the values were added to Excel (or calculated there), the representations may be different even though they look the same.

This is another instance of a more subtle problem. You can't truly trust any data that comes from outside your program. The real world always seems to be messier than it needs to be.<shrug/>