Single-sourcing is a procurement strategy, not an outcome of RFP manipulation. When a purchase is put forth, the end-user[1] writes the specifications, and procurement staff[2] determines the procurement strategy. If the end-user can convince procurement staff that only a single provider can meet their needs, then they move forward with a single-source procurement.

When a specification is written specifically for a single vendor, it's not called single-source; although the end-user probably advocated for single-source during negotiations with the procurement department. Not surprisingly, this is a constant source of friction for procurement departments. End-users almost always want to buy their preferred solution, and especially in IT. There are often good reasons, but I digress.

In some contexts, the single-source procurement strategy is simply not available; usually due to procurement law. This is when you most commonly see manipulation of specification. However, competing vendors can challenge the procurement in court through a procedure usually called a "bid protest". That's not exactly what's happening here, but it's similar.

1: The person who will ultimately use/implement the purchased goods/services.

2: Usually a separate department responsible for ensuring that goods/services are "responsibly" procured.