If someone is very junior or has some other discount factor (in practice that's the only real one), you'll probably go lower than expected for a 30 year old with a PhD, freeing budget to bump up someone else.
And sometimes, you get a stellar CV with enough brand on it to make the staunchest McKinsey reconsider his limit policy, so whilst you can't put the number on the ad, you can warn the candidate, get his number, and take a punt on management in case it might work.
The strongest factor for flexibility is when you have a chance to affect the outcomes for the company, but need to prove it before you can claim the pay raises. Then, it makes sense to take somebody at under their market rate, and 3 months later have a chat with management and bump up.
Hiring is a really sensitive topic because HR is often the biggest budget. I once seriously weakened my political position by attempting to automate away "Excel blue collar workers" in another department, which would have lowered the budget and therefore relative power of their boss internally. Those with the most flexibility in hiring (startups, which have no established internal territory to worry about) also have the least resources...