And this is why minions don't work. When is a hit point not a hit point? When it belongs to a minion of course.
While you may feel the minion rules don't work, and you are not alone, many of us do feel the minion rule works, and works quite well.
The design of 4E is very different than the other versions. The lvl 21 Legion Devil minion is designed only with the concept of them being used as a group (around 4 per PC) against an appropriate lvl party. The designers gave no consideration of how a single Legion Devil minion fights a commoner. In fact, none of the monsters are designed for combat of any sort against another monster or NPC. These monsters are challenges for the PCs and only for the PC, any other use is for plot device only. Some of the monsters can be player races in some cases, but then they would be PCs not monsters.
A party, or even a single PC, should never encounter a single minion all by himself. It would be "bad" encounter design according to the game. Now, a DM may create a cool encounter based on a lone minion, but that would be the DM's story doing the work, not the game mechanics.
According to the game design, if the party is to encounter a single opponent, it should be a "solo" type monster, not a minion.