Zeta was the sixth letter in the Ionic alphabet, which eventually became the standard alphabet, but some early regional alphabets had the letter digamma (which looked like the Roman F and was in the same spot). In the Greek numerical system, zeta equaled 7 (digamma was 6).
With Greek numerals, the first 9 letters were the numbers from 1 to 9, the second 9 stood for 20, 30, etc., and the third 9 stood for the hundreds. That required using three obsolete letters--digamma, qoppa (equivalent to our Q, immediately following pi), and sampi (a sibilant, following omega). The number 111 would be represented with rho-iota-alpha. It makes the Roman numerals seem easy.