In 1920, Lord Grantham says that Sybil, when she dies, is "24 years old" and by the time of her season, we know she's a August/September (she dies here in 1920) to December birth.
6x03 is May 1925 (we know because of the stamp that Spratt is looking at!), meaning that Mrs Hughes arrived sometime between June 1894 and May 1895 to have been there, as Thomas stated in 6x03, for thirty years.
Yet, in an earlier season - either 2 or 3 - Mr Carson says Mrs Hughes never knew Sybil as a baby. Yet, if Thomas's words are right, then Mrs Hughes would have been there between 14 and 3 months (14 if she started in June 1894 and 3 if she started in May 1895) as Sybil was born between August/September and December 1895
By this, Mrs Hughes would have known Sybil as a baby. So, is Thomas wrong, is Lord Grantham rounding, is Carson wrong, or is it something else?