Trading Profit – Basis Period, Chapter 6, page 39

Hello sir,
In example 4, The first period of account was from 1st December 2020 to 31st May 2022, and counting the months between these dates would give 16 months, please is there any explanation as to why it was counted as 18 months in the answer and in the lecture video 6.3.

Also after solving it the way I assume correct using 16 months, I wasn’t able to get the correct answer doing that “answer check trick” of deducting the overlapping profits from the assessed profits, which must be equal to the total adjusted trading profit of the trader in the question, on the contrary using the 18-month first period of account verifies ? that check.

Please I need your explanation on this issue.
Thanks.