The reason is that you probably bought your first subscription (non auto renewal) directly via Zattoo shop and the second one via iTunes.
If you already have an active subscription which was purchased via Zattoo Shop and then you buy a subscription via iTunes (in order to proactively renew the first one) the second subscription will not follow the first one. This means that the existing subscription will not delay the start date of the second subscription. Both will run parallel. You received the following message from Zattoo on your iOS device: “You may purchase a new auto-renewing subscription at any time, but it will not extend your current one-time subscription (...)”.