Discrepancy in number of years of full contributions

Good evening My national insurance record in the HMRC app says I have 27 years of full contributions, with no gaps in my record. However, the list of years shown below adds up to only 24. Why can there be a discrepancy, please I was contracted out for several years, in case that can be one reason. Thank you very much for your answer

Hi,
Depending on your date of birth there may be credits applied for some tax years prior to the ones listed that make up the 24 years.
Being contracted out as you have stated can increase the number of tax years required to obtain the full state pension.
You can call the HMRC National Insurance helpline or use the web chat services in your personal tax account to discuss the tax years with an advisor that make up the 27 years, also you can speak to the Department of Work and Pensions regarding the state pension forecast and the qualifying years that go towards it, if you are close to six months of retirement you will need to speak to the Pension Service instead:
HMRC NI helpline:
Number - 0300 200 3500
Overseas - +44 191 203 7010
Open - Monday to Friday: 8am to 6pm
Closed on Saturdays, Sundays and Bank Holidays.
Web Address - www.hmrc.gov.uk/ni
Pre 6 months to state pension age – DWP Pensions forecasts
Number - 0800 731 0175
Overseas Textphone - +44 (0)191 218 2051
8:00am to 6:00pm, Monday to Friday
Over 6 months to state pension age – State pension helpline
Number - 0800 731 0469
Welsh - 0800 731 0453
Open - 8:00am to 6:00pm Monday to Friday, closed bank holidays and weekends
Northern Ireland
Enquiries - 0800 587 0892
Thank you.