This is usually handled differently between various providers, but you could say that the monthly price is a standard. By using a straightforward monthly price you won't get a straightforward hourly price and vice versa. However going from a hourly price to a monthly price means that you get prices like $7.3 or $14.6 assuming that we multiply with 730 with is the average amount of hours in a month.
Now the interesting part here is that some providers are using the monthly prices to give you a discount if you keep your server over the end of the month but for us that's not really a proper approach for a cloud with hourly billing. Hourly billing means hourly billing and shouldn't be more expensive just because you don't keep your server a full month.
While all the billing is now based on hourly prices we're still working on the listed prices which isn't accurate currently, some are multiplied with 672 and some with 730. Prices listed here: https://www.99stack.com/pricing are currently correct however, both hourly and the new monthly approximated prices which will be listed together with the hourly prices everywhere in the future.