Different tyres can, and do, have difference rolling resistences.
Some car tyres are marketed as being especially energy efficient.
I would have expected the front to have an effect as well, but maybe less so.
Different tyres can produce different readings on the same rolling road dyno, same day. How worn the tyre is can affect things as well. I.e more or less transmission losses.
Having said all of that I'm quite surprised you are seeing such a big difference though.
You may also be seeing a mileage error creep in over time. What happens as the tyres wears is that the speedo/odometer tend to over read as the tyre reduces in size, it might only be a few millimetres but it stacks up.
The only time I worry about fuel consumption is when the warning light comes on and I've no idea how far to the next petrol station