it is not weird - that's how it works.
When you create a New Issue, a new Item is created with next available Item ID. This Item ID # is used to assign issue ID.
Between your issues 1 - 9, you must have altogether 14 updates. Every time when you update/edit an issue a new Item is created and data from last item (aka Current Item) is carried (copied) into it (that's what you see on Edit Page). This new item gets the next available Item ID (and carryover the same Issue ID as read-only).
To observe this behavior, create a view and include both IDs (Issue ID and Item ID). Remove the Current=Yes filter. Next you can (a) sort them Modified Date (not By Modified - the person) and you will see all Item IDs lined up, or (b) Group the View by Issue ID and and sort them by ID.
Using above you will be able to trace all IDs. If you still see some IDs missing then it simply means you have created and deleted Issues. If I recall correctly, lists tracks the last IDs used (means, IDs are not reused if records were created and deleted)
In response to another post about using Issues List as helpdesk tool and a response in favor of Infopath. With WSS Issue list you can get a good mileage with no desktop overheads (as Infopath requires a client desktop install). Don't limit yourself to default install of Issues List, you can do all sort of creative things with it to build a real application. Personal Disclamier - I have not used Infopath but that what I understand about it (re client install)