A developer or tester might be good at specifying how severe a bug is: impact (does everyone get it, or just some users) multiplied by frequency (does it happen in commonly used functionality?) multiplied by a category factor (is it a crash, cosmetic, dataloss...) will give you the severity.
Some completely different person can come along and give this a high priority because of business reasons, and to prioritize fixing the bug compared to other functionality. Two bugs with the same assigned severity might impact two different markets where the business knows one is critical for the survival of the business, and one is already a safe market. Then it's a priority to fix it.
A second reason might be due to ROI (easier severe bugs are better to prioritize than difficult bugs of the same severity).