There was a theft from poloniex almost a year ago. A percentage of BTC was stolen. So poloniex subtracted from everyone's balances the percentage and finally payed back after some months all the coins from the fees.
I am just writing this in case you are interested and want to research it more.
The amount stolen (77
BTC) was below the cutoff, so this won't be included. A cutoff provides an objective way to include or exclude thefts.