I tried to read as much as I can from this thread. I understood how each subsequent key has a bigger interval than the one before it, but I don't understand how the highest keys were also spent?

If this was the hex for the 57th key :
0x00000000000000000000000000000000000000000000000001eb25c90795d61c

which I assume is a random between (57bits)1FFFFFFFFFFFFFF and 100000000000000.

How were the highest outputs spent? Wouldn't they be extremely hard to find because of the massive 256-bit max range? I don't get it..