Why would they strike millions of coins then overstrike each one? No, as Martin says, it's the die that gets recut either to reuse it or to correct an error or to recut wearing characters.
Thanks Mike. It's hard to determine a VIP proof when there were normal proofs in that year. Why wouldn't they issue VIPs with early strikes of an 'ordinary' proof set? In any case, the real determiner - the penny - is the normal proof issue, as has been pointed out.
Why is Tony Robinson coming to mind?
Mind you, that other advert for a grossly overpriced gold coin does quote the name Pistrucci .. wonder who that's aimed at?
Estimates are simply the auction house's "finger in the air" which may or may not be based on past experience. Also bear in mind that some are "come and get me" estimates, set artificially low to encourage a bidding war.