taxation BPP 112

Hi there, I have a question on BPP 112 about class 2 NICs calculate simon's income tax liabilities and his class 2 and class 4 national insurance contributions for the tax year 2022/2023,if he is treated as carrying on a trade in respect of the disposal of he freehold house. I'm not sure why the answer is 3.15*19weeks. From the question, Simon disposed his freehold house on 27 August 2022, from this date to 5 April 2023 should be 7 months, why the answer is not 3.15* (4*7)weeks instead? Thank you in advance for your help.