If you're talking about Estimated Bill, we can figure out how Railway got to that by looking at your current bill. Your current cycle started on October 31st which means its been about 5 days since then, during that 5 days you've used 5usd in usage. This makes it easy- you're using about 1usd a day and therefore in a month you'll use around 31usd which explains your estimated bill being 31usd.

I'm not sure why your individual services cumulative services' estimated bills don't match up, my guess is that you forgot to show one of your projects or you've deleted one. Another reason why I think that is because if you add the current cost of all the projects you show together then it comes out to about 4.5usd so 1.5usd of usage is missing.

If you deleted a project recently then you'll estimated bill should adjust over time, I estimate your bill will be around 23.5usd if these are the only 3 projects you have now.