Ch 4 Tax Adjusted Trading Profit - Individuals Example 1

I hope this is not a repeat question. I don't understand how non-business expenses make it into the profit calculation so that we have to add them back to tax those. Shouldn't have they been not allowed to be included in the accounts in the first place? For example why do Jeremy and wife's rent and utilities appear in the accounts? I would have expected to include only the relevant proportion here which is also the entire tax allowable part. Thanks in advance Zeynep