This math is based on having 100% of the existing hash power (the network is 500H/s, you would need 500H/s) - in most cases you would probably purchase enough to give yourself a decent buffer above the existing hashing power. The numbers are meant as very rough estimates to give people a better idea of the costs.
With so many actors it could be very steep. Like 51% capacity has 99% chance of success and 49% has 1% of success and 40% has practically zero chance (these are made up numbers).
It could also be quite shallow as in 1% capacity gives you 1% of success... that would actually be quite bad and invite malicious actors to test the system with regular attacks.
It's a race of who can get to N blocks first. The higher N the "steeper" it is. N is set by the merchant so as to balance security with quick transactions