A local PR Agency has contacted me after they think they have made a ‘boobo’ with regard to a part-time member of staff’s contract. Question is

Her holiday entitlement is “25 working days” which for her equates to just over 6 weeks holiday! This should have been pro-rata to 20 working days holiday.

Can they explain the error and legally reduce this from 25 to 20?
Lynn Tulip