the diff will be more clear if u go for an example of Low
priority high severity  and high priority low severity 
1. low p high s :- suppose we have a link on our site that
is visited very rarely may be in once 1000 times on which
our system crashed that is High Severe because whole system
crashes but Low priority because Link is very rare.....
2. misspelled company name or missing logo from front page
have no impact on s/w functionality but having a worst
impact on company's goodwill so this is this case of high
Priority but Low severity........
 i hope u will understand...