The chances are that you have added the rows, and subsequently deleted them: SQL doe not "reuse" numbers in an IDENTITY column so when you delete rows that you get gaps in the sequence that will not be filled.
That's by design, so that data in other tables which refers to the "main table" via the ID do not associate the wrong data with the "new" row, even if a formal FOREIGN KEY relationship has not been established.
If you need a sequence that always has no gaps, then look at
ROW_NUMBER (Transact-SQL) | Microsoft Docs[
^] - but that does not uniquely identify a row, except in the context of a specific SELECT statement and should not be used as an ID for that reason.