Why doesn;t the system send the next mail on the exact time as when registered when I use wait one day?

A client is complaining all the time because she doesn’t receive the next mails on the next days at the same time as when registered, when I use wait one day.
I signed her up at 5 pm, the next day the mail was sent on 5.40 pm, the day after it wasn’t sent even after 6 pm. Why is this? And can I see when it will be sent?

Greetings,

Lucy