The Outstanding Tickets screens will very rarely match the Pre-Invoicing report exactly. This is due to them looking at completely different parts of the system.
The Pre-invoicing report looks at Transactions while the Outstanding Ticket screens look at jobs.
Example:
You complete a skip job for £300.
Then the next day you realise there should have been an extra charge of £50 so you add an item to the transaction.
The result is that you then have a job for £300 showing in the OST screen, but a transaction (that will be on the pre-invoicing report) of £350.
You would have a similar situation if an item was removed from the transaction or if the prices on the transaction were amended at all you would end up with a mis-match. Any changes made to the transaction will never retroactively make the same updates on the original job.