I just wanted to chime in to say that straw bedding should be avoided as much as possible because it can get moldy inside and cause aspergillosis (respiratory) otherwise known as "brooder pneumonia".
The area you pictured where they're hanging out looks like straw that's older and a bit musty. We all tend to expect mold to be obvious, but it can be subtle, and with birds delicate respiratory systems they have less tolerance than mammals.
The IB you mentioned was / could still be a good guess, but do they have the tell-tale wrinkles on any of the eggs? If not, I would think aspergillosis.
That said, I do not believe that people should compare numbers. Life is complicated, animal husbandry is sometimes bizarre.
Everything can be so different from one set-up to the next, starting with the climate, including the products and choices you have available (like feed, I aim for 20% protein minimum), and on it goes.
Some people have wonderful success, and many happen to be careful, knowledgeable caretakers. That doesn't mean that any caretaker who is careful and knowledgeable will have the same success. Just like doing everything right in a pregnancy doesn't always mean a healthy baby.
The local breeder I got my Orpingtons from quit keeping them because his coops are in the sun and they fared poorly. I did a lot better at first with my shady coop.
Some of my breeds are a lot hardier than others, until the weather changes and the ones who thrived in summer fail in winter, and vice versa. One breed that was very resilient in terms of disease resistance (cocci) and hot weather, also yielded two individuals who broke their necks flying into stuff they shouldn't have, in a setup where many other chickens have been fine.
Despite a Ft. Knox brooder setup with two protective broody hens, I lost a chick overnight two weeks ago when a pencil (pencil!) thin corn snake managed to find its way in and constricted the poor baby who was so much bigger than it. I made it out in time due to the noise that the chick was still fighting, but it was wrapped so tight around the chest and neck in a figure 8, and I couldn't get the snake separated enough from the chicks head that I could exert force on the dang thing, and sadly the chick suffocated before two of us managed to get the thing off (and killed it, since it could come right back and was so slim as to fit through the slightest gap, plus I was very mad).
The other day, we lost our rooster to some kind of crop issue we were trying to treat but just would not resolve.
After 13 years of keeping chickens, and working with horses and on a horse breeding farm before that, my conclusion is that there are bad seasons and good ones.
You can (and should) try really hard to solve any problems you become aware of, eliminate as many dangers as possible... but you can't control life and death absolutely. Trying to nurture life also means witnessing more death, as heartbreaking as that is.
The solution with animal husbandry then is to have more animals than you would need if they were all successful (but not so many that the numbers create problems).
although the vet just called to tell us Penelope made it through the night so right now I am a bit happier 
