If you have the Inventory calculation turned on in your forecast, then LivePlan is using your accumulated Direct Costs entries to predict future inventory re-orders.
You may notice, however, that in the final year of your forecast (Year 3 of a 3-year forecast or Year 5 of a 5-year), your inventory balance doesn't change, even if your direct costs are increasing.
The reason behind this is that LivePlan's inventory calculation relies on future direct costs. It uses them to create inventory orders to meet those upcoming needs. In the final year of the forecast, however, the app isn't able to see (or predict) what your direct costs might be after the end of the forecast. So instead, the app defaults to calculating from the last available direct costs - those from the final year.
If your projected inventory will be higher than what LivePlan is calculating in that final year, here's a useful workaround: you can enter the additional inventory as a Current Asset entry. The amortization of that asset will represent the gradual use of the inventory.