Most of those pitch decks are "after the fact" fabrications, not the actual original decks. I'm afraid this resource is nearly useless, even if I admire how much work went into it.
Don't think it's quite as dramatic as you imply here, and I don't think anyone is deliberately fabricating slide decks in order to trick you.
So, I look at a a couple, but quickly get the feeling that these may not be genuine, that this site might be theonion of pitch decks, or something. I go to HN comments to see what others think.
I was disappointed to find that the author of the site apparently doesn't really know if these are real or not. I'd like to see more than a passive aggressive "oh gee thanks this was a fun project 2nd year business student blah blah I'll look into it", when the site clearly claims to show me “Facebook's original pitch deck” as the FIRST one which is very obviously not genuine.
Comparing these decks to the decks I have in our tracker (a dataset of similar size to yours), there's a lot of differences. Even if you only take the pitch decks of the successful companies (most of which we didn't invest in), a lot of these decks look a lot more polished.
Here's what I think is going on: Some decks are undoubtably real seed pitch decks, some decks are demo day decks from accelerators, and a few of these decks (Facebook's, for example) are after-the-fact business school exercises from students.
So, I do think most decks are real, but what's tripping up some people is how polished some of the decks are. True pitch decks used to get a seed round or into an accelerator aren't very polished because you're constantly changing the deck based on feedback over the span of 20-50 pitches. The truth is that many investors I know worry when they see a too-polished deck that everyone else has already seen the deal and passed :-P.