The manual is suggesting you set it at 10° for an inservice valve adjustment, just in case you're a futz with the chain.
Mr. 4129R is correct that crank marks are not particularly accurate, but then, neither are the cam sprocket marks. To be precise, you need to find the actual, exact TDC and then use the valve timing specs, a dial gauge, and special venier sprockets or offset woodruff keys. This can make a huge difference on a full race engine. Not so much on a stock engine.