I don't think they are allocated sequentially (and there aren't that many unique...

I don't think they are allocated sequentially (and there aren't that many unique cards/versions). But IIRC Gatherer's using the IDs from the internal "Multiverse" DB, which also includes all the brainstormed card ideas that never made it into an actually-released set, which would tend to inflate the numbers.

Edit: and they are not allocated sequentially. These two cards have only ever appeared in the 2014 core set, and have sequential collector numbers in the set but not sequential IDs:

http://gatherer.wizards.com/Pages/Card/Details.aspx?multiver...