So the figures published would have to be a guess.
No, they are accurate for what they measure. The rest of the hidden wealth would be a guess, but if its in paper, Polish, money they can count that accurately as they know the amount in circulation. What you can't know is the amount in foreign currency, gold etc, that's where your guesswork comes in. The 60% figure is guesswork, those people probably don't use banks becuase them are penniless, an equally valid guess.