I do agree; as all of you, that posting a journal entry on the inventory account directly from GL, or posting a document in Inventory with no "post to GL" will absolutely result with an imbalance IV versus GL. Although, this is a business case and need to be considered through policies and procedures in order to prevent doing so, and the system supports such rules (by locking the IV account from GL not to allow account entry but from modules).
On the other hand, what should be taken into consideration is other scenarios that might generate variances. I do believe that one of the main reasons that primarily causes a difference between Inventory and GL is the "Cost Adjustment ". There are several cases in which the cost adjustment document that is created in Inventory doesn't result with 100% accurate IV-GL balance.
The good news is that GP 2013 provided a very elegant tool that could provide you with a detailed report for IV-GL variances. Although, understanding the root causes behind these variances is a must in order to eliminate any further potential variances.
I would be definitely happy to discuss further details on this issue, in case your GP has "Cost-Adjustments" scenario that might be "partially" the root cause beneath your variances.
Best Regards,
Mahmoud AlSaadi