They're not supposed to accept your offer.
They'll accept the best offer. If one doesn't go through, they may--or may not--accept the offer that (in their opinion) is second best. And that's not always the second highest.
$100,000. 20% down. Good credit history. Minor contingencies.
$98,000. 10% down. Weak credit history. Major contingencies (such as selling your present house).
$93,000. All cash. No contingencies; buying in "as is" condition.
A lot of people would choose that third offer, even though it's $5,000 less than yours.
A seller can choose to accept, reject, or counter any or all offers.
Hope that helps.