Future Pensions systems mismatch

I live overseas and after submitting a CF83, HMRC have (7 months later) sent me a shortfall statement: class 2 contributions would be relevant for 10 years from 2006 (overseas and working) while class 3 would be relevant from 2016 (overseas and not working). This is exactly as expected. My pension forecast shows I currently have 25 qualifying years and based on the current contributions (all pre-2016), I need to find 6 more years to reach the full pension amount. My understanding was that because I only have 25 years currently, I would be able to find 5 years from the pre 2016 years (all class 2) with one year of class 3. The Future Pension Centre said no - I must pay 3 years of class 2 and 3 years of class 3. Why?

Hi lemonpress,
The Pension Service will advise the tax years to pay voluntary National Insurance contributions to improve your State Pension entitlement as paying some tax years will not improve it.
Please see contact details here:
Contact the Pension Service
Thank you,