There are two possible contributory factors here - the OS grid reference is inaccurate, i.e. wrongly measured or recorded, or not precise enough, i.e. the number of significant figures provided in the grid reference is insufficient.

Many of the current milestone grid references in the National database were produced many years ago when the only method was direct measurement on Ordnance Survey maps. These early surveys adopted 3 figure references as this was the maximum precision that could be achieved by direct measurement.The majority of the 3 figure grid references provided are correct as far as they go in terms of significant figures. However, they are not very precise because 3 figure references only give positioning within a 100 metre square

Conversely,there are many instances of inaccurate grid references as the result of a wrong reading or transcription error that to date have been difficult to pick up until the Google Earth mapping presentation.

Today, there are tools such as SatNavs and other GPS position devices that can provide much more precise references and we are now standardising on 4 figure grid references as a result.