This doesn't make any sense, why would they make you pay less to make it open access? You mean it was 1200$ more?
– "normal" publishing (behind a paywall) is free for authors,
– the open access option was set arbitrarily to $2000 by the publisher, and they decided to offer a discount at $1200 instead.
(For conferences with proceeedings, even normal publishing is not free - part of the conference fee is to pay the publisher.)