As you will appreciate sowing/planting times* can be quite different across the UK due to ones geographical location, this difference can be as much as four or five weeks between the north coast of Scotland and the south coast of England.
To cater for this anomally the writer has grouped the tasks in groups of four, where the first box might suit those in the south and the fourth box for those in the north.
If a grower finds that he/she can do these tasks say a week earlier, or indeed a week later, i.e. at a time to suit their own particular circumstances, then they could mentally place an imaginary box in front or behind the printed boxes and work to these dates.
* Generally 'late frosts' are the governing factor in determining when these can be done, and this subject has been addressed here;