It may not be possible for Plusnet to give you any exact details due to GDPR against your referals, but it may be something like one of them with a value of £1.25 has left Plusnet or hasn't yet paid their bill for that month.
Another possibility I suppose (though I've never heard this one mentioned) could be that one of them has moved their billing date so that it now falls after yours, whereas previously it was before. I would have thought that would cause a one month discrepancy in them.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
