I enabled the update feature that emails me whenever the site has an update available. I noticed that the time the email gets sent is always incremented by one cron run. For example, I have cron running hourly, so if there are updates needed it will email me at 8am, the next day at 9am, the next day at 10am, etc. But, based on the math, whatever interval cron runs at the email will likely always be one run later.
In hook_cron, the if statement requires the current time to be greater than interval (in my case, 24hrs). If it is exactly 24hrs, it will not pass.