Severity is how the reporter sees the problem: if it interferes with their particular use case, it's of the highest severity. Almost all bugs end up being of the highest severity.

Priority is how the developer (or, more likely, the project management team) sees the bug. Highest priority bugs are the ones in the project manager's spreadsheet and ended up there because of the most vocal complainers (paying customers, an inconvenienced CEO, etc). The rest will never get fixed.

It's impractical to use the severity of the bug to rank the priority, because they're not strongly correlated.