Valve Clearance ???

[Pilotdave]

I've been reading various sources for valve clearance specifications and have observed:

Service Bulletins (p220): .013" - .015"
Les Andrews: intake at .013" and exhaust at .015"
Bratton's Catalog (p2): .010" - .012"

Which leads to several questions:

1. Why are there so many different "specs"?

2. Are the variations just another example of the (sometimes) delightful ambiguity that's a part of this hobby?

3. What might be the most appropriate spec?

Thanks!