Some ideas…
– Apple is not the most valuable brand in the world… Actually, It’s the brand with the highest value… which also depends of how big the company is… (GE is one of the brands with highest absolute value, however, in relation with the company market cap, the brand value is, in percentage, quite low)… So it’s not about how valuable your brands are but how your brands are valuable 🙂
– The three companies use similar approaches (when working with a client)…. Variations are coming when they make their rankings which only use “back of the envelope” information… So variations are more related with the sort of data that each branding company is using
– In the cases of ranking, probably, the best use we can do is to analize results comparing year after year… because mistakes are done systematically… in the annual comparisons, those errors are annulled

Just a week-end thought 🙂