by IneptVagrant » Mon Jul 13, 2015 8:22 am
I can think of two reasons, there is probably others.
First, you're client doesn't connect to all available peers/seeds. There may be firewall issues, or network config issues or the seed is on another tracker which you don't have access to, and is not using DHT. So you receive the parts via a 3rd party. Seed to some other peer, to you. As the 3rd party completes they log off, and you continue getting parts from other 3rd party peers.
Second, the seed is using slot shaping. They only advertise the least available parts of the file exist. They will have the full file, but mask parts of it which change occasionally and since they don't advertise they have the full file they aren't counted as a seed.