I don't have an answer to this but it is very frustrating for the recipient as they can confuse the icon with the date of the invitation. It is still present in 10.8.2.
I can't find any other references to this problem on other fora.
I'm getting the exact same thing. Tues 9. Very odd.
I hope this is a bug and that it will be fixed shortly.
(in other words I hope it's not a case of an Apple designer dropping a nice shiny and completely misleading image in an email notification template. Usability fail)
I doubt this is a bug, and more a bit of laziness on Apple's part. Just as iCal used to show a fixed date for its icon in the dock a few years ago... Apple eventually made it reflect the current date.
In these invites, Apple needs to replace the fixed icon with a dynamic one that actually represents the date of the invite!!