It's random, and apparently not intended. I saw Nutaku say on twitter that they are fixing it.

Yeah, there's a note in the game updates about it. Says that there's going to be compensation to all players based on how far they've gotten in the event so far.

And here's what's on the PeroPero page.
◆Information
We are currently revising the structure of the event and improving the situation with high level boss fights. All users will be equally compensated based on their event progress. We apologize for the inconvenience

That sounds like it's probably a good thing, hopefully the bosses HP actually scales bettter with the SED of the person who found it. I mean don't think that EVERY player should be able to handle EVERY encounter by themselves otherwise it really defeats the purpose of summoning friends or perhaps buying pudding if they want all the EP for themselves. But eventually everyone gets to the point where they're summoning the very hard to kill 8.7M girl and are forced to rely on higher SED friends. Hell even the weaker girl that I posted was way too much for the person who summoned it to handle without using MUCH pudding.
-EDIT-
Actually that guy in that thumbnail could have handled it BUT some people are getting ridiculously out-classed by the Kasumi's they're finding!

According to the JP wiki, the bosses were supposed to follow a set scale of HP according to their level, which I posted in the event guide. If they had been doing so, the levels of HP would be much more manageable even with our low average SED.

Yeah, this whole mess up with the scaling is annoying. Hope they'll fix it soon because i keep getting those damn 8.7m bosses and i ain't no spending gold on elixirs anymore.

Either i have gotten to a point where i cannot get lower health enemies, or RNGesus hates me. Not getting anything but those damn 8.7m ones, no point in even asking others to help with them -_-

Same here. I'm getting those 8.7m guards way too often.
A couple of normal HP guards, but most of them are those 8.7m.

I wonder if they'll use the next update to add those pending event cards.

So annoying. All I get now are the 8.7mil bosses.