I wouldn't worry too much about it.
They are going off what your bank statements for last year/this year say, and not what you actually have at the moment.
Next time you send them a statement they will go from what this last one said to what that new one says.
(I don't think they usually look at all the ups and downs, just what it says at the bottom of each statement they do have).
It looks like as I said they have decided that your savings have dropped by just over £1k since the last time they calculated it (last year) and so they should now be paying you an extra £4 a week.
If your savings do go up or down again by next time they look/calculate then they'll simply adjust for that.
And never forget that the people who type these letters often seem to be dyslexic and make typing mistakes.
Plus they will often re-use an old letter and forget to change things like numbers.
Technically then yes you should tell them each time you savings change by £250 up or down, in practice they are not too bothered as long as it's updated at least once a year and doesn't go over £16K.
Of course if it's a large one-off change, say you get a couple of thousand in or out, then you should tell them straight away.
It probably costs more in wages for someone to look at and recalculate if they did it regularly than the change in payment would be.
Say it costs £10 wages time to recalculate, fill in forms, type/send a letter, etc., and they did it every month that would be £120 a year or £2.40 a week, which is probably more than most payments would change by.
If they want to see statements more often then they will ask for them.
If you want to tell them more often yourself then maybe give them a ring every 3 or 6 months to report a change of savings?
But most people would prefer not to stir them up and just wait until they ask.