It'll be to do with the fact of 13 being the magic number kept the same, as say if 10,000 tickets are available and each season more and more people hit the magic 13 and above mark the number that 10,000 goes into becomes less.
Below as an example:
First season 5,000 people have 13+ everyone gets Kop tickets that wants them.
2nd season 6,000 people have 13+
3rd season 7,000 people have 13+
4th season 8,000 people have 13+
It lessens the chance of getting Kop tickets as the number should rise 14,15,16 each season but that creates a closed shop effect and lessens peoples chances in the free for all by not doing that.
But the club keep it fair in maintaining the magic 13.