Catherine's birth date has been pushed significantly earlier than older pedigrees assumed. This matters a lot. Sally Varlow's peer-reviewed 2007 article, Sir Francis Knollys's Latin dictionary: new evidence for Katherine Carey, examined Francis Knollys's own family records. The journal's abstract explicitly says these newly discovered records strengthen the argument that Katherine was an illegitimate child of Henry VIII born during his affair with Mary Boleyn.

So this is not merely internet genealogy. It was published in Historical Research, the journal of the Institute of Historical Research.

There is a contemporary 1535 statement that Mary's other child, Henry Carey, was regarded as Henry VIII's son. John Hale, vicar of Isleworth, reported that Mr Skydmore showed him "young Master Carey" and identified him as the king's son by the queen's sister. The original survives in Letters and Papers, Henry VIII, VIII, no. 567.

That statement concerns Henry Carey, not Catherine, so it cannot prove Catherine's paternity. But it is important because it demonstrates that the idea of royal paternity of a Carey child existed contemporaneously, rather than being an eighteenth- or nineteenth-century invention.

There is also the strange pattern of royal favour to William Carey during precisely these years. Anthony Hoskins's detailed genealogical study identified a concentration of Henry VIII's grants to William in 1522–26, including major grants around the probable births of Catherine and Henry. A particularly conspicuous grant came in February 1526, only days before Henry Carey's recorded birth.

The interpretation that these grants were compensation to a compliant husband is plausible, but the grants themselves never say that. They cannot be treated as proof.