Because the button on the webpage invokes the same command that the crontab does. The issue is, that when you add a crontab, it is actually added to the users actual crontab on the system. However, when run manually, PHP tries running the command. Because the code is encrypted, I can't say as to why it doesn't work, but generally PHP uses "exec, system", or one of many commands to perform operations on the system. Generally though, PHP needs you specify full path (Ex: /bin/bash /full/path/to/script.sh), whereas crontab can simply execute the script directly.
That's why I mentioned checking the log. If you don't see anything in those logs, check /usr/local/cwpsrv/logs/ to see if there is anything there.