Crontab determines when the Magento Cron Job will run, based on the configured settings. This job refers to the Magento “cron_schedule” and invokes all the processes/tasks that fit in the duration of 5 minutes (since the last run of crontab). The default settings to run the job is every five minutes.
The job also deletes older entries and inserts new rows for new tasks, as per their schedule defined in the respective module’s config. file. In logicbroker's case, our module is configured to run every minute. This means that when the crontab executes, our module will be called 5 times (1 for every minute). Inside our module we have controlled execution based on values defined by the user in the admin screen. If its set to 10 minutes, then the module will check its last run date (preserved by us in `core_config_data` table) and run accordingly.
When you execute the cron.php file manually, it does not execute logicbroker's module script instantly, but follows the above process--therefore, you may have to run it multiple times to have it run successfully and meet its time.