When you create a job and set the payrate, sometimes the payrate changes to something else when it's sent out to the candidates. This is because the option 'Overwrite payrate with candidate payrate' on Step-3 of the Create New Job wizard is ON. So, the assignment takes the payrate set on the Candidates' profile instead of the one set on the job. This option is generally used when you have candidates at a different pay rate.

If you want this setting to be OFF by default, you can also turn it OFF from Settings >> Settings >> Jobs and assignments default settings >> Overwrite job payrate with candidate payrate. You would still have the option to turn it ON on the Create New Job wizard if you want to switch it ON on some jobs.