I've convinced myself that this happens in gmail / hangouts history search too. It'll very confidently tell you that here are the only six results for your search term going back to the beginning of time, but if you go and manually dig up something that you know is there from ten years ago, then all of a sudden there are seven results the next time you search for the same term. I haven't done this methodically, and I…
Can confirm. Google, the search company, has lst the ability to accurately search even the data that it holds, never mind any other.
Google, the ads company
FTFY.