Any final salary benefits you built up before 1 April 2014 are linked to your full-time equivalent pay, even if you are part time (as your membership would have been reduced accordingly).
Any benefits you build up from 1 April 2014 is linked to your career average revalued earnings which is based on the actual amount that you earn. If you are part time, this will be the part time pay.
If you disagree with the pay figures disclosed on your statement, you should contact your employer's payroll section for further clarification.