callouses on heal and toes

My names Austin, I work as a river guide in the summers and from snow melt to snow fall I wear a brand of sandals called Chacos.
There the best product out there as far as comfort and stability goes for outdoor activities but myself and other wearers have a problem with bad callouses at the back of our heels and somtimes on the fron of our toes.
This is strange because under my foot where the sandal actualy touches my foot, there arent any callous problems.
Its only on the back of my heel right above where the sandal touches and on my big toe in the front, again, above where my foot makes contact with the sandal, that I get the callouses.
They get so bad that they split and sometimes bleed.
Even before I start working on the river and my feet go from wet to dry all the time, just wearing them around town gives me the calloses.
I don’t have problems with calloses with other shoes except with, just where I wear my chacos all the time.
What I can’t figgure out is why I would be getting the calloses the way I do where the sandal isnt even directly touching my foot.
I already know how to take care of the calloses but what I want to know is what would cause them to form that way.
Is it the foot bed drying out my feet, keeping them too moist or could it be something with the texture of the sandal, its hardness or softness?
The sandals are made of Polyurithane.
If you could tell me whats going on, Id appriciate it.
ThanksAustin