Additional tax owed via self assessment on earnings via PAYE

Completing self assement for the first time. I'm full time employed and have included details from my p60 for this (my paye account says nothing is owed) Self employed I only started this tax year so I have zero to add total this years assesment. I have added the little bit of interest earned. The self assesment calculation is showing a owe a small amount but can't work out why?

Hi,

If you are unsure why an amount is due, please contact our helpline 72 hours after submission and we will be able to view the calculation and advise.

Self Assessment: general enquiries

Thank you.