That is hard to answer definitively without an in‑depth audit. The Auditor General might be able to answer that question more specifically.
On the other hand, I can say that National Defence does its own expenditure profile, with cabinet approval of course. One might expect that the person doing the planning would have a very good idea of what will actually be spent. If there is a significant gap between actual and planned expenditures right from the first years of the plans, one wonders whether the planning was done with due diligence, especially if there was no extraordinary event that disrupted planning, such as the COVID‑19 pandemic.
There might be room to improve the planning, but as I said officials with National Defence and the Auditor General could probably provide a more specific and detailed answer.