http://www.sypron.nl/idgaps_txt.html
Why "identity gaps" occur
Identity gaps can occur following a rough server shutdown ("shutdown with nowait") or a crash of the server process. This is related to the algorithm the server uses to generate identity column values: essentially, a counter is kept in server memory, holding the identity value most recently issued. When a new row is inserted, this counter is incremented and the resulting value is assigned to the identity column in that row. While the new data row itself is written to disk, the new value of the in-memory counter is not. Only when the server is shut down in a normal way, this value is saved on disk.
This algorithm makes the identity feature very fast, because no I/O is required to generate a new value. On the other hand, should the value of the in-memory counter be lost, as is the case for a server crash or a "shutdown with nowait", it is not possible to continue at the next identity value, because the last issued value was not saved anywhere. In this case, the server will continue generating identity values starting at some much higher value - which is what creates the identity gap.
Exactly at which higher value the server picks up, is determined by the configuration parameter "identity burning set factor", which, to a certain extent, can be used to limit the maximum possible size of an identity gap. However, because this a server-wide setting, it is not possible to apply this to individual tables.
See the ASE System Administration Guide and Technical Document #20113 at http://www.sybase.com/detail?id=20113 for a detailed description of how to use this configuration parameter.
In view of these underlying technicalities, the risk of running into identity gaps could be seen as the price one has to pay for the high performance offered by the identity column feature. Also, it is clear that this risk cannot be completely excluded. DBA’s should be aware of this, and be prepared to perform recovery procedures.