Whilst the Whitworth was undoubtedly an accurate rifle for its time, setting standards for other gunmakers to aspire to, contemporary reports can be misunderstood. The system used to measure precision of rifled arms was the "Figure of Merit" (FoM). It is a measure of mean radial distance of shots from the centre of a group. Today, contemporary results of FoM are from time to time reported as group size, presenting misleading information and unreliable reports of accuracy. The actual size of the original group cannot be determined from the Figure of Merit, however for a 20 shot group multiplying the Figure of Merit by about three and half will give an indication of the likely overall size of the group.
So in trials in 1857 the Whitworth was recorded with a FoM of .37 feet (or 4.44 inch) at 500 yards - based on the above this would be a group size in the order of 15.5 inch (roughly 3 minutes of angle). At 1100 yards the FoM of 2.41 feet was recorded - so a group size in the order of 8.5 feet.
See the following article and videos to which it links: Measuring Accuracy