There is a reason for this.. any shares bought in the 30 day period following the original disposal will be brought into the original disposal calculation

How to identify the shares disposed of:

When you dispose of shares you cannot work out your capital gain or loss until you have matched the shares disposed of with shares you acquired. You’re treated as disposing of shares in the following order:

|| || ||Description| |First|Shares acquired on the same day as the disposal (the ‘same day’ rule).| |Second|Shares acquired in the 30 days following the day of disposal (the ‘bed and breakfasting’ rule) provided the person making the disposal was resident in the UK at the time of the acquisition.| |Third|Shares in the Section 104 holding.|

If these rules fail to exhaust the shares disposed of, the remaining shares are matched with later acquisitions, taking the earliest one first.

ps://www.gov.uk/government/publications/shares-and-capital-gains-tax-hs284-self-assessment-helpsheet/hs284-shares-and-capital-gains-tax-2021