There could be a number of things it is, but let’s assume it’s a ‘whole of life’ policy which carries a number of options from the outset. If this was set up years ago it could have been on a minimum, balanced or maximum premium. In short some policies had larger monthly premiums at the outset so a ‘fund’ was built up to offset against future increases as the policy holder got older. Some had small premiums at outset, with a larger rise due years down the road. Most went for an in between premium ‘balanced’ where you got the cover but laid a little extra to help offset future rises.
It’s quite hard to explain this in a short post as there’s so many variables, but I’d imagine your mum has some variation of the above.
Does your mum now need the cover? Can she afford the extra premiums? What is the surrender value? When is the next review date as the premium could increase far more or the cover could reduce substantially? What’s her health like?
Just a few things to consider and discuss with your mum.
One question to ask your mum - she would have taken that cover out for a reason at the time, is that reason still valid? I’d never advise anyone to cancel a life insurance policy (for obvious reasons), but I would ask people to think about why they have it and if they need it.
P.S. that drop doesn’t seem a lot which is a positive.