I change the token and log in. How?

Question

JSON Web Token

Hello.
I use firebase and its built-in authentication.
I try to receive the data request http://firebaseexample.com/blabla/.json- I receive Permission Denied.
If I do

http://firebaseexample.com/blabla/.json?auth=jwt_token

then I get my data, everything is fine as it should be.
Now about the strange
thing. My jwt-token is for example blabla.blabla.blablaC0x, where the last three characters are taken from the real situation. So if I replace the last character "x" for example with "t", then I get an error "error": "Failed to validate signature." . If I replace it with the character "u", then I get this error again. But if, for example , I replace the last character with "z" or with "y", then I don't get any errors and the data is returned to me. Question: what's going on?