It is likely that four examples of the V rated tyre were not available at the time and rather than make you wait until one could be found the tyre fitter decided to provide an otherwise identical tyre but with a higher speed rating. Given that this is the only difference between the tyres it is unlikely to be noticable in terms of handling or ride quality and should cause you no issues. For reference, the V means the tyres are rated to 149mph while the W indicates the odd tyre is rated to 168mph, so you should be safe regardless.