IDENTITY can give you gaps, and if gaps are a concern, you should not use IDENTITY. You use IDENTITY (or sequences in SQL 2012) to improve concurrency. Because IDENTITY is not transactional, there is no serialisation.

Explain to the customers, that these numbers are completely arbitrary and that gaps of all sorts can be expected. And you should not waste your time on finding out why these gaps occur, because you already have the answer: you use IDENTITY and asked for it to happen.