Apple care - product registration
When a reseller sells Macs, they are supposed to report the sales data properly back to Apple. If this data is not reported, the "sales date" in Apple's system is the date the reseller bought it, not the date you bought it from them. Apple is requesting your original proof of purchase, which you've provided.

In general, if it is clearly shown that you have attempted to purchase AppleCare within your one-year period you should have no issues getting this registered, even if the date creeps slighly past the one-year mark. A good option as well is to visit a local Apple Store if you have any issues, with any documentation. I have seen store Geniuses and Managers go out of their way to ensure issues like this are resolved appropriately.