How come my call time usage on cellular usage does not add up to my calling log?

If you have a data plan, or other options, that may affect your billing.

Had you discussed this issue with the cellular provider? 😐

Did you reset your usage to match the dates your carrier uses? How much difference is there? Also, many cellular carriers round usage, so if you were on the phone for 30 seconds, they count a minute, or other differences. The carrier should be able to answer your questions.