In researching this topic this morning, I was reminded of something that I almost forgotten. The standard explanations involving ring currents have come under question in the last twenty years or so. Empirically, the downfield positions of these protons is obviously not open to question, but the reasons behind the downfield shift may not be what standard textbooks say.
Chaitanya S. Wannere and Paul von Rague ́ Schleyer, ORGANIC LETTERS 2003 Vol. 5, No. 5 605-608.