If the maximum speed limit is 70Mph and you're travelling at that speed in the middle lane,
The issue is, you may be doing 70mph indicated with a 10% over estimation on your speedo.
The car behind you may be going 70mph indicated on a 5% over estimation, therefore going faster than you, you are therefore holding him up and should move over.
Moving to the left when able is a requirement, not a request.