How do you know this? Being arrested and interviewed under caution are two separate things anyway: if you're arrested don't the police have a time limit in which to charge you, but interviewed under caution means you are free to leave the interview at any time and may have access to legal advice?
There is a formal procedure for a voluntary interviewee who is not under an arrest in which the interview is under caution, but it is also the case that when an arrestee is interviewed, that is under caution (and the caution will normally be re-read to the arrestee at the outset of the interview).
Normally when people say 'interview under caution' or 'interviewed under caution', they mean a voluntary interview. NGB1066 did not exactly use that phrase, if you look at his post. It looks like he was referring to the generic idea of somebody interviewed following arrest, but it is possible that the information NGB1066 has been given is wrong and in fact Julie was not arrested.