First thing that occurred to me (as an ex forensic accountant) is that someone has kept your payroll account open so that they can pay themselves extra amounts without it getting picked up by anyone. THe payroll says its you, and the tax is paid over correctly but the net pay goes to the fraudster.
All hunky dory until your combined pay (actual + ficticious) exceeds the higher rate band and you get a tax bill for the difference between teh PAYE amount and 40%. I've seen it happen.
One lot of tax people asked for details of my earnings over a 2 year period, I didnt have them so I asked which ever tax office handled my employment, they told me via letter that I had earned xyz at some company I have never heard of, i passed this on to the people with the original query, 2 months later I get a 500 quid tax rebate
I gave all my correct NI numbers etc, so who knows what happened