so went to bar last night, and put my drinks on my debit card like usual

well this morning i'm looking at my online statement and it shows that i have a $100 pending transaction at the bar i was at last night, but the receipt i have in my hand from the bar shows $57 ... wtf?

i've never had that before, are they trying to cheat me here or is it like a pre-authorization that will clear out because on the CC receipt it does say offline transaction