Monster hit points are affected by: (a) rank; (

difficulty; © level. Level 26 NM Orsino has a lot of hit points, yes. The math is correct, there is nothing 'funny' with the game.
Please note: your assessment is based exclusively on your intuitive impressions (was your log enabled? have you calculated damage done to him by your party? which level were you? what difficulty setting were you playing on?), while mine is based on actual knowledge of game mechanics. When experimenting with Spirit Warrior archer builds back in DAO/DAA era, I was getting a lot of hatemail from people incompetent in game mechanics (a 5k crit in DAO/DAA was more or less what a 217k crit is in DA2).
If you really wish to discuss it, open the 2da.rim, extract the .gda tables, look up creature properties. Simple as that. With all due respect, your gut feeling is a poor indicator of damage math being 'funny'. Or of anything else, for that matter.