Acetylacetone contains two types of enolizable H-atoms -- six on the methyl groups and two on the central methylene group -- and the questions was which one is more likely to become involved in tautomerization. Of course, you figured this one out: The methylene H-atoms is more acidic. Enolizatio in that position has two advantages. A conjugated systems is formed and there is the possibility for the formation of a hydrogen-bond in a six-membered ring.