It doesn't really but it seems to because of the way percentages are worked out using the total mix of oil/methanol/nitro. If you look at that table where 5% oil gets added there's 67% methanol and 15% nitro so they're in a ratio of 4.47:1 (which is what the needle valve is tuned to) and after adding extra oil the table says there's 64.48% methanol and 14.44% nitro which is still a ratio of 4.47:1 and all that's needed is a slight "richening" of the needle to account for the extra volume of oil.

An easier way to think about it is if there was a 50/50 mix of methanol and nitro. No matter how much oil you add there's always still a 50/50 mix. Even easier is a fuel with no nitro. With any amount of oil you still tune just for the methanol going into the engine.