Sometimes there's a little play in the valve rocker arm. When you put the feeler gauge in to check one tappet clearance, the arm will shift a little and give you a false reading. So it's good to have two feeler gauges of the same thickness which you insert at once. Then adjust both valves while both gauges are inserted.
If you do remove the inspection plug to get at the end of the crank, great care must be taken to only very lightly tighten it back up when you close it back up. Or it'll be stuck.