UK tax past paper Dec 2020- Aurora

Hi Sir,
UK tax past paper Dec 2020- Aurora
Q (b) the Principal parivate residence relief: Sample Answers:
Why it needs to add another 1.5 year( 18 month) for the Private residence period?

I cannot find anything regarding 1.5 years and think we should just use the PPR years shown in table given in the question?

Thanks.