Some exmplanations:
- You may have included the file in some code,
- Someone is accessing the file (but you outed that)
- There may be another cron job defined (it may happend if for some reason two jobs were created or someone created it by other panels, by hand with different times) - like for testing.
Someone may execute it manually
- you you must find out yourself.
- block it by password or totally with .htaccess
- check your cron settings (also for each user crontable)
- check logs
Also - WRITE some code into you php file that will log HOW and WHEN it was executed - that will allow you to further test what is happening.