Yes, usually a result of the individual agent's relationship with the hotel that matters here and not the (host) agency's; though being affiliated with an agency which regularly sends business to the hotel may help. It's also not entirely uncommon for agents/ICs within one agency to help each other out.

And yes, I'd agree that negotiating a confirmed late check-out is easier at city center locations. And easier to negotiate something like 2p.m. than 4p.m. - but with any negotiation / request, it helps if I have some leverage (e.g. a one-week booking at $500/night vs. a one-night stay at $200/night).