A credit score can change from one month to the next without the person doing anything differently. The movement comes from how the underlying data is collected and refreshed rather than from new behaviour.
Balances are reported on one day
Lenders send account information to credit reference agencies periodically, typically once a month and usually on the statement date rather than the payment date.
The balance recorded is whatever was outstanding at that moment, so somebody who uses a card heavily and clears it in full can still be reported with a substantial balance.
A single large purchase made just before the reporting date can therefore move a score, even though the money was repaid before any interest arose.
Time passes whether or not you act
Several factors in a scoring model are functions of elapsed time, including the average age of accounts and how long ago a missed payment occurred.
Those quantities recalculate continuously, so a score drifts upward as a clean record ages and adverse entries move further into the past.
Closing an old account can push the average age down sharply, which is one of the few ways deliberate tidying makes a file look worse.
Reporting is not synchronised
Each lender reports on its own cycle, and there is no common date on which everything updates together.
A score checked on two consecutive days can differ simply because one account refreshed overnight and another has not yet done so.
The same file also produces different figures at different agencies, since not every lender reports to all of them and each uses its own model.
The models themselves are revised
Scoring models are rebuilt periodically as the relationship between past behaviour and future repayment shifts across the population.
When a model is updated, the weight given to particular factors changes, and a file that was unchanged can be scored differently under the new version.
Different lenders use different models and generations of models, which is why an application outcome does not always match the number a consumer has been shown.
What the number is actually for
A score is a summary of a file at a moment, produced to rank the likelihood of repayment relative to everyone else in the population.
Lenders combine it with income, existing commitments and their own history with the applicant, so the score alone does not determine any decision.
How scores are calculated, what may be recorded and for how long are all set by rules that differ by jurisdiction and change over time.