It's about supply and demand. If he really is the best person for the job, then there's nothing to be gained from paying him a higher fixed salary. But if there's someone else who's even marginally better that you can get for $10M, it's probably easily worth it - for the size of the fund, one person's pay is a negligible amount.
That's the fundamental problem of hiring: you have to make educated guesses about who is going to make a better employee. And you should be willing to pay more to hire someone who you believe is better. For some companies, salaries are such a small part of their revenue that it makes sense to offer some candidates a lot more... and if your first choice happens to accept a lower salary, it doesn't really matter - you've saved a drop in the bucket.
