My understanding was each supporters club got allocated so many tickets based on their membership, longevity etc. Any supporters club not taking up their full allocation would then return them to the club.
If you're a small club with less then 10 members, it's quite possible you got allocated a few tickets but none of you are going.
Similarly some are quite a distance and will have been sent tickets. We've got 4 OSC in South America and one in Hawaii, it's a hell of a trip from there, and 15 in China (including Wuhan) and one in Mongolia.
There were 30 guys from the Ukranian supporters club at Kharkiv on the third trip, I doubt any of them can go, and we have a supporters club in Moscow, but knowing City they were probably sent some acces codes!