Statspack snap Id's

I run statspack from a batch job daily on all databases To do this I have to edit the sprepins script to specify which snap id's I want the report to be based on. I regularly find that the allocation of snap id's varies. Sometimes the id will be incremented by 10, sometimes by just 1. For example:-

NAME      SNAP_LEVEL    SNAP_ID Date/Time
--------- ---------- ---------- -------------------
COBRA              5      24965 09.08.2011:11:01:33
COBRA              5      24966 09.08.2011:11:16:48
COBRA              5      24975 09.08.2011:14:01:55
COBRA              5      24985 09.08.2011:14:16:55
COBRA              5      24995 10.08.2011:11:01:35
COBRA              5      24996 10.08.2011:11:16:50
COBRA              5      25005 10.08.2011:14:01:57
COBRA              5      25006 10.08.2011:14:16:57
COBRA              5      25015 11.08.2011:11:01:38
COBRA              5      25016 11.08.2011:11:16:53
COBRA              5      25025 11.08.2011:14:01:59

The outcome of this is either the report fails as it can't find the correct snap id (it looks for max(snap_id), and max(snap_id(-1) or (-10), depending on the value specified in the sprepins script) or else it generate a report which covers several hours and is therefore meaningless.  My question is why does the allocation of snap ids change in thsi way. Oracle version is 10.2.0.5.9, standard edition.