In case anyone else had the same question, I received a clarifying response from the auction house (see below):
"We will take your bid as you bid in the room. If you bid 1000,- for a lot what is estimate with 500,- and we have no bid you will get it for 450,- (90 %), if there is a bid in the book for 600,- you will get if for 650,- (always + commission). If you will win a lot"