This is my guess and not necessarily what "really" happened at Microsoft...
Does anybody remember the first Ultimate Games on Demand sale on Xbox Live? It was fairly underwhelming with a few decent titles. Kameo, Bully, Dark Void, Perfect Dark Zero. Subsequent Ultimate Games on Demand sales had much better deals. I daresay some of them were even great deals. I am guessing Microsoft studied the sales data from the first sale and used it to pitch to other game publishers and that is how they got stronger titles for later GonD sales.
By the same token, perhaps Microsoft studied the data from the early Games with Gold deals and used it to negotiate better deals like Sleeping Dogs. PS Plus wasn't that great either at first. In June of 2012 Plus was relaunched with the Instant Game Collection but before that we got Sega Genesis titles, Minis, PSOne games that were 15-18 years old, etc. That was what Plus looked like for nearly two years before it relaunched with more recent games. I'm sure Sony also studied the user data for a long time before coming up with the Instant Game Collection. Microsoft could be doing the same. Too early to tell; it depends on what they offer over the next few months.
For the record- I don't care how old a game is on Plus or Gold. I only care about whether I own it and if I want to play it.