'The underpayment relates to the "old" state pension system - affecting those who reached pension age before 6 April 2016 - which had special provisions for married women.

Under these old rules, married women who had a poor pension in their own right could claim a 60% basic state pension based on their husband's record of contributions.'

https://www.bbc.co.uk/news/business-56275415

What took the DWP so long?