That's not exactly how it works. .com and .net domains are automatically renewed between the Registry and Registrar when they expire. This adds one year to the Registry Expiry Date. If they remain unpaid by the Registrant after 45 days, the domain goes into Redemption and the Renewal is refunded by the Registry to the Registrar. At no time does the Registrar Expiry Date change unless the domain is actually renewed by the Registrant. Hence. Every expired domain which has not been renewed by the Registrant shows the wrong expiry date at the Registry for .com and .net domains. And so, the extentions anyone is wanting most to capture, show the wrong expiry dates at the Registry.
The auction is only an afterthought by those money-grabbing registrars who either 1) Run their own Auction House, or 2) Have and agreement with a 3rd Party Drop-Catching service like SnapNames or NameJet, to auction off domains which have not been renewed before they go into Redemption after the 45 days, when they lose control of the domain after that 45 days. This is why, in most cases, the maximum number of days a Registrar will give the Registrant to renew a domain is somewhere between 30-42 days. Depending on the Registrar, and their modus operandum.
The Registrar Expiry Date will always be correct because the Registrant hasn't renewed it.