Project available Funds vs Requisition available Funds

Summary:

Project available Funds vs Requisition available Funds

Content (please ensure you mask any confidential information):

Why is it that when a project budget is created and Requisitions, Purchase Orders and Invoices are created from same project budget. When you check the Funds available from the project side which is to add the committed and actual cost and deduct same from Project budget, its not the same as when you go to creating a requisition using the same project budget and go to check funds, the amount available differs.

Why is this and how to have both reflecting the same available budget?