No nines in Algorand addresses (Vanity Address)

Just a trivial curiosity, but I’ve seen that the number 9 never appears in Algorand addresses. Is there a mathematical explanation for this? Are there any other impossible occurrences?

I realized this while playing with vanity addresses

Algorand addresses are encoded using base32 which is a standard 32-character set using the twenty-six upper-case letters A–Z and the digits 2–7. The digits 0,1,8 and 9 will not be found in any properly formed address.