Ann: Appendix 4C - quarterly, page-40

I'm at a loss, so if one of the smarter folk on here could help me I'd appreciate it.

When the company I work for, creates an invoice to a client, that's counted as income when we get the cash is irrelevant as that is cash flow. We list unpaid invoices as a liability as they 'could' go bad. If someone doesn't pay up we have to credit note the original invoice and effectively record 'negative' income.
So if OBJ invoices P&G for sales, why do we have to for them to pay it weeks later for it to be realised as income?