Hi Richard,
Do you have any closed years in the GL? Are you filtering on year or something like that?
If you are not filtering in any way, then you will get tons of results for the historical records in the GL30000.
There can also be issues if this was an environment that has been running since before the HITB was a report, or if the HITB reset has been ran. These clear out records in the SEE30303, and although we do add balances so it all matches, the transactions prior to the reset will not show 1-to-1.
As for answering your actual question, records get added upon posting the Inventory Transaction. If users delete or don't post the GL transactions this report will not know that, and this can lead to balance issues.
Hopefully this helps you nail down what is happening.
Thanks!