Is cellular data usage normal while on Wi-fi?

When I'm at home for a day or so, my iPhone is always on Wi-fi, but cellular data is active as well.
As far as I know, all data should go through the Wi-fi connection, and cellular is just there for a backup.
Still, even on a day like that, my provider shows that many MB of data is used. When I do the same with cellular switched off, cellular data usage is zero.

Is there a way to solve this without either buying more data or always having to switch the cellular data toggle? Is this normal?
Thanks!

iOS 6.1.1, iPhone 4S (jailbroken, but currently all tweaks uninstalled)