What happens is that a particular item goes on your tax return in a different year to when it goes on your accounts.
The reason you do this is because tax law tells you that you have to do it.
The main examples are:
Depreciation - Accounts rules say you have to estimate how long the asset will last and depreciate over that period of time. Tax rules say that you depreciate at 18% reducing balance if you expect the asset to last less than 25 years, or 6% if you expect it to last longer than that. There are different rules for cars where the tax depreciation rate depends on its CO2 emmissions.
Share options and bonuses - Accounts rules say you put them in the accounts in the period where the staff-member did the work to earn them. Tax rules say you put them on the tax return when the employee receives the money and pays income tax on it.