It's more complicated than just unpaid items or deletions. Rollbacks after inserts also leave "holes" in sequences.

Because smallserial, serial and bigserial are implemented using sequences, there may be "holes" or gaps in the sequence of values which appears in the column, even if no rows are ever deleted. A value allocated from the sequence is still "used up" even if a row containing that value is never successfully inserted into the table column. This may happen, for example, if the inserting transaction rolls back.

Afaik the best we can say about sequences is that they're monotonically increasing (if NO CYCLE is used (which is the default)).

The id is not the item count. It's a rough overestimate of the item count.