This phenomenon is already explained here Make keeps changing final numbers in a variable module - #3 by samliew
In short, your number is too large. You need to store it as a string instead.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!