The inventors, patent practitioners and everyone else involved in the project of trying to get a U.S. patent are under a duty of disclosure to the USPTO of everything they know of that might form the core of a rejection. There is no requirement to do a search - it is what you know. The duty to disclose exists from the day of filing until the patent goes abandoned or is issued.
The exact criteria of what to send in and what not to send in is not 100% clear. Until a couple of years ago patents were being invalidated by courts because something wasn't sent in that would not have made any difference if it had been sent it. Under that threat, people sent in anything they knew of that was remotely relevant. The patent office themselves would rather only get the on-target ones to make their lives easier.
Then the examiner does one or more searches. All of the inventor supplied and examiner found references are listed on the front of a patent as things that were considered in the granting of the patent. Of course if any single reference or combination of references made for a rejection that was not overcome, then the no patent issues.
It may not be entirely logical but when I see dozens of references on the front page of a patent I start thinking it must be solid because of the volume of things that didn't knock it out.