Can somebody please explain this?

Last night I put an order in to Back a horse (Neville) in the Wincanton 15.55 . £50 at 8.4. I placed this through the Betfair interface. No third party software involved.

This morning I checked to see if my order had been filled. It had but at the reduced odds of 6.11.

Now  I know there were 2 NR's in this race but I was still surprised to see my bet matched at a lower price than I had asked.

Is this normal protocol.

Thanks for any input.