I recently verified a degree, and the university reported that the degree was awarded five years after attendance. In other words, the student attended from 1995 to 1998 but the degree was not awarded until 2003, five years later. Why would there be a gap between the time the student attended and the time the Ph.D. was awarded?
(Note that the years above are fictitious and the actual years were at the time of the war in Vietnam so it is possible that some kind of military service may have been involved.)
In many universities, attendance (in the sense of registering for courses and/or paying tuition) is not required for Ph.D. students once they have completed the coursework and residency requirements for the Ph.D. After thus period, the Ph.D. student can work independently with minimal (or no) supervision from the Ph.D. advisor. When the Ph.D. thesis has been written, it is usually submitted to the advisor who does the first evaluation as to whether the research is of adequate quality, and then to the Ph.D. committee (examiners), the thesis defense is scheduled. After the defense has been successfully completed, the thesis (which often undergoes revision based on the Ph.D. committee's comments and questions that arose during the thesis defense) is submitted to the University and the degree granted at the next commencement/convocation etc. During this last period that begins with the thesis being submitted to the Ph.D. advisor, the Ph.D. student might need to be registered at the university and possibly have to pay some tuition (usually not the entire fee for a full term), but this is usually not regarded as attending the university or listed as such on CVs and job applications. In short,
it is quite possible to have a gap between the last date of attendance and the Ph.D. degree award date.
To add an alternative answer: until recently, Oxford did not allocate graduation dates, it was up to students to sign up for a graduation ceremony, to graduate either in person or in absentia. For various reasons, there are a small number of people who didn't officially graduate for many years after passing their course.