Other news outlets may have different wording, but if you look at this one from the BBC, what they have said about what he has admitted to is actually quite ambiguous and suggests - perhaps - to me that he has admitted to an additional charge of raping a second victim sometime between 2 and 10 March. I've snipped and bolded for clarity but haven't excluded anything that could change the sense. It is definitely unclear though, as 'second charge' may mean 'subsequent to the first charge which is kidnap' or may mean second charge of rape. If the latter, it would explain the issue with the dates.
Sarah Everard: Wayne Couzens admits rape and kidnap
"Appearing at the Old Bailey via video-link, PC Wayne Couzens, of Deal,
pleaded guilty both to kidnap and rape.
"The court heard
he also accepted responsibility for Ms Everard's death but did not enter a plea on the charge of murder.
"He
pleaded guilty to kidnapping Ms Everard "unlawfully and by force or fraud" on 3 March.
"He
also pleaded guilty to a second charge of rape between 2 and 10 March."
JMO