make draft P30 correct

Draft P30 did not show correct employment allowance claim for one employer. Was told by support that it would be correct only once finalised. However other employers show the correct amount in the draft P30 reports so why cant ALL employers show correct amount in draft. Reports need checked before they are finalised. that is good practise. Mistakes in reports create a lack of confidence in the system, draft or not. How do i know its not an error that is going to be carried forward to the final outcome.