Why one has to be careful when interpreting DWP rules, my pension was one for a state run industry, the SERPS payments (the difference between contracting in and out), were not collected by DWP, e.g. a lower NI contribution was made by me. Therefore the pension is reduced due to this non-collection.
My assertion of deductions is made based on a quote from DWP on what I will get on retirement, I'm not quite there yet.
Other Private Pension Plans could have different agreements with DWP on the handling of SERPS contributions.