Difference in differential backup sizes

We are observing a weird behavior in the differential backup sizes. The backup of a database which was taken on Thursday was of size 25 GB and on Friday it's just 2 GB and today it's 19 GB. What could be the reason behind it? We can't use these backups for recovery as they are failing throwing the error the differential backup cannot be used for restore because the database has not been restored to the correct earlier state.

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