It's like this:
Beginning balance $1000
Debit transaction 1/1 $5 Posted 1/5 (overdraft fee)
Debit transaction 1/2 $1 Posted 1/5 (overdraft fee)
Debit transaction 1/3 $4 Posted 1/5 (overdraft fee)
Debit transaction 1/4 $800 Posted 1/3
Deposit 1/4 $1000 Posted 1/6
Debit transaction 1/4 $201 Posted 1/4 (overdraft fee)

Overdraft fees collected: $140 vs. if they posted the transactions in order there would have been NO overdraft.

They deliberately post larger transactions first in the hopes of dinging you extra in overdraft fees.