Unwarranted overdraft

My account went into an overdraft despite me checking my balance prior to every purchase to ensure I have enough money in my account. A debit account should not be able to go into an overdraft. I spent 3 hours with their customer service only to have every agent end the chat without providing me with an actual answer. They all say its because the merchant banks the amount at a later date but my balance goes down at the time of purchase and i do not have any loans or debit orders coming off of this account. So imagine my surprise when I logged into the app only to see I have -400.

No one can explain to me how my pending charges show up on my statement, the amount for a purchase is deducted immediately yet im somehow still in an overdraft. It should not be possible for my funds going out to be higher than my funds coming in, yet that's what my account is showing.