It's a rounding effect: each punch duration (or line) is rounded to the nearest minute, but the total is the sum of each unrounded punch and only rounded afterward. As a simple example, in format MM:SS, if there were three punches, 01:41 (one minute, forty-one seconds) and 01:36 and 01:01, they would each be rounded to 02:00 and 02:00 and 01:00. If you sum the unrounded numbers, it's 04:18, and then if you rounded that, it's 04:00. But if you add each rounded number, it's 05:00. So the total is actually more accurate, then summing each line.
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