In minecraft, how does armor and the unbreaking enchantment work, specifically?

Question

Alright, let me break down what I'm talking about, and why I'm asking. In minecraft, while experimenting with extremely high enchantment levels on armor, I've discovered something strange. I have a pair of diamond boots with the following enchantments:
Protection 32767,
Fire resistance 32767,
Blast resistance 32767,
projectile protection 32767,
unbreaking 32767,
thorns 32767,
and feather falling 32767.
Now, having unbreaking 32767 would suggest that the boots would probably never break in my lifetime, but for some reason that isn't the case. After just an hour of going about regular minecraft activities, I've noticed that I've lost over 75% of the boots' durability. Two things I've noticed that take more durability than I'd expect: Explosions, and standing in lava both visibly drain the durability at a surprising rate. Why am I able to lose durability if I have unbreaking 32767?

Answer 1

Now, having unbreaking 32767 would suggest that the boots would probably never break in my lifetime

is incorrect.

How Durability works:

For armor, (60 + (40/(Level+1)))% chance a use reduces durability (each durability hit against "unbreaking" armor has a 20%/27%/30% chance of being ignored). On average, armor lasts 25%/36%/43% longer.

from Minecraft wiki

Therefore, no matter how high your Durability level is, the percentage of losing one durability point must be over 60%. Specifically, with Unbreaking 32767, the chance to reduce one durability point is approximately 60.0012207%.

Just for reference, Fire Resistance, Blast Resistance, Projectile Protection and Feather Falling will not affect durability point loss.

As mentioned by SpiceWeasel in his comment, using the {Unbreakable:1} tag when summoning an item will make that piece of gear completely immune to damage. Refer to the NBT tags page for more information.