Private residence & letting relief

Hello tutor, I hope you’re doing well. I have a query with regard to Chapter 20: CGT – Relief for individuals- Test your understanding 3 – from Kaplan study text.

Ques: On 30 Sept 2023, Eleni made a gift of a house to her grandson, Kostas. The house had been bought by Eleni on 1 Sept 2010 for £45000 & was extended at cost of £10600 during 2011. A market value of £140,000 at 30 Sept 2023 had been agreed by HMRC.
Eleni occupied the house as her main residence until 30 Sept 2019, a period of 109 months. During this period, Eleni rented out 1/3rd of the house for 96 months. The tenant did not share any living space with Eleni.
Eleni then went to live with her sister for the remaining 48 months, until 30 Sept 2023.

*Calculate the chargeable gain after reliefs arising on the gift.*

Solution:
Working: PRR & Letting relief

Total 109 months (-) 96 months let = 13 months occupied full residence

**Lived in full house (actual occupation) = 13 months (EXEMPT)
**Lived in part of house:
2/3 actual occupation = 64 months (96 months let *2/3) (EXEMPT)
1/3 let = 32 months (96 months let *1/3) (CHARGEABLE)
** Living with sister (for 48 months)
Last 9 months = 9 months (EXEMPT)
Rest of period is chargeable = 39 months (48 – 9)

My query is:
With regard to the “living with sister” part, I’ve understood the last 9 months out of the 48 months being EXEMPT. But why is the remaining 39 months completely “chargeable”, when 36 months (or 3 years) of absence can be availed for the exemption rule of “up to 3 years of absence for any reason”?
That being said, the answer would change as follows:

** Living with sister (for 48 months)
Last 9 months = 9 months (EXEMPT)
Up to 3 years of absence for any reason = 36 months (EXEMPT)
Rest of period is chargeable = 3 months (48 – 9 – 36)

Kindly clarify my query as soon as possible. Thank you.