Even if domain name expire, previous owner have some smaller time period when he can renew domain even if it was auctioned and higher bidder won.
For example, if I win auction at Godaddys aftermarket, previous owner has still 7 days to renew, in that case, my $ will be refunded.
Doesn't happen often as if previous owner didn't see or ignored few warning e-mails months before expiration, he'll probably even miss that short period.
In your case, this happened so domain has it's previous owner and you got backorder option again.
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