Steering within 10° either side of your course is good, but after 60 miles you could be 10 miles off course and after 1200 miles you could be 200 miles off course. In the real world tides turn, winds shift and helmsmen wake up. Errors balance out and the course you finally make good is usually pretty near the course you wanted.
Be aware that when tacking, you are zigzagging across your mean line of advance and errors made on one tack are cancelled by those on the next tack, but on a run you hold the same course for long periods and errors can accumulate.
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