I think it’s human nature to predict based off our own interpretations and experiences though and analysts are just feeding off each other’s thoughts.
There is no real accurate measurement how analysts gather all their data, because there are so many sources out there. Many of them are bogus claims just for the sake of creating controversy. Aside from that, using our own anecdotal information based off what we see surrounding us is also very minuscule in theory, because we don't fully have a wide enough spectrum of how a particular product is doing.
As Far as Apple announcing their own sales figures, even if you want to believe that they are massaging their numbers, that's the only credible information we have to go off and or believe. There is no other source besides the source themselves.