D&D experience oddness

I'm looking at a very tough encounter in the introductory adventure, Keep on the Shadowfell. It's a 6th level encounter when the PCs are very likely 1st level... it's known net-wide as a TPK (Total Party Kill) encounter. No explanation as to why it's so tough is given.

The DMG says that a hard encounter is three or four levels over the party's level. This one is five levels over. Granted, none of the individual monsters are over 3rd level, but still.

But that's not what's bothering me right now. What's bothering me is that this encounter is worth 1250 XP. But two 3rd level encounters are 1500 XP total. Combined, two 3rd level encounters are easier than one 6th level encounter... assuming you get to rest in between, your Encounter powers refresh and you get to spend healing surges. You're more likely to use your Daily powers in the bigger encounter than in the two lesser ones.

Trimming the 3rd level encounters down to 625, so that they're exactly half the power rating of the 6th level encounter doesn't change the basics... because XP are a flat measure, that is, five kobolds serially are worth the same XP as five kobolds at once, there is no value in taking additional risk. As the risk goes up, the rewards don't. Why take the extra risk with no extra reward? The ideal situation is to isolate individual monsters and kill them solo, five-on-one, because that's worth exactly as much as killing them in groups of twenty or thirty at a time.

Now, granted... the players don't always have a choice about how powerful an encounter is. But that seems like weak sauce to me... "You gotta take the higher-powered encounters because that's all that's being offered." And I don't think that's how the game is meant to be played... but I'm thinking that players should be rewarded appropriately for the risk.

My thinking is that XP values should be a function of the difference between party level and encounter level. The further away from the party level the encounter gets, the more it should be worth above its base value.

When it comes down to it, if I throw my players into this encounter, and they come out on top, they're going to throw things at me if I hand them a measly 1250 XP.