Good question.  It’s because we used to have two types of State Pension: the Basic State Pension and the Secondary State Pension (formerly SERPS).

You probably didn’t receive a Secondary State Pension because, until April 2016 self-employed NI contributions didn’t count towards it. However, your wife’s Employee’s NI contributions, did count towards her Secondary State Pension – so she gets a bigger pension.

Class 2 and Class 4 contributions have been, in effect, just another tax (so are Employer’s NI contributions).

Since April 2016, we have had one flat-rate State Pension irrespective of how much NI we pay and the self-employed are now treated the same as the employed.  A bit late for you, I’m afraid.