Yes, that is how they work. Sequences are not guaranteed to give a gapless list of numbers, partly because a rollback of a transaction that used a sequence value does not return the value to the available list, and partly because of sequence caching.
That should not be a problem for a non-meaningful id.
There is a long discussion of this here: http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:530735152441
