Why are numbers are being added into the PIN field automatically?

If the Clock PIN page opens and there are **** in the PIN field without anyone typing anything in, your browser likely has auto-fill enabled. Browsers that are set to remember passwords will automatically fill this field in. This is particularly common when using Chrome

Set your browser to not remember passwords for Jackrabbit. Please check your specific browser's help documentation for more information on how to disable password saving.

If you want your browser to remember passwords, you set the Clock PIN page specifically to not remember passwords. This is usually found in your browser settings.