The Custom Format you're applying relates to the datetime value that XL is generating... so the summation of time to: 790:49:15 is as far as XL is concerned is the equivalent value to the following date: 1st Feb 1900 22:49:15 ... thus when the time summation cell is formatted as m "Months" d "Days" h:mm:ss "Hours" you will get 2 Months 1 Days 22:49:15 Hours because you're formatting the Date value itself rather than the time value as a difference if that makes sense at all ?
Perhaps you want something like:
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