Its possible that specific members have received more low quality exchanges then others. Tickets were selected in the order they were created. The tickets of the most active members stayed longer in the system (they are in low demand since most members already sent stamps to them) and when a new member sent stamps, most probably it received the address of one of the top 10 most active members. It seems that many low quality exchanges come from new members.
I have changed the way tickets are selected when sending stamps. Now they are completely random. This means that new members will send to anyone. This will not resolve the low quality exchanges, but they will be more evenly distributed among all members. Receiving one low quality exchange from time to time might not be such a big issue as when receiving several in a short time.