Correct initial timing?

Hey everyone and trailbeast lol.
Upon looking up some info in my motors manual I happened to see that the timing spec on a 68 340 is at tdc and 69-72 is 5 degrees btdc. Im running 15 degrees of initial timing and car runs fine. Is there a reason why the factory timing is so low?
Although different the 351 in my bronco is at 11 degrees btdc so 5 seems low to me.