Some of the map markers are not manually placed on the actual location but geocoded from the postcode. In a densly populated area this can get you many yards wrong, typically putting the marker on the wrong side of the road; "over a mile" sounds unlikely, though, unless there are only a handful of houses between you and the pub and you share the same postcode.
Why do you care, anyway? Do you get people knocking at your door asking for a pint?