I am predicting Apple gets close to $100B in sales this quarter. People have already forgotten Homepod will be in the mix as well as The 8, X, and Watch 3. They did almost $80B last Q1 and people expected doom because of "weak" iPhone 7 demand.
I seriously doubt Apple gets anywhere near $100B in sales. Don't forget, Tim Cook had up-to-the-minute preorder figures on the iPhone X as well as store-by-store inventory allocations for the month of November which would account for substantially all sales for the quarter. He also had the full month of October sales for the iPhone 8 and the number of phones to be shipped from Foxconn before the end of the year. The only real variable would be walk-in orders for the iPhone X during December, and if there were a substantial number they likely couldn't fulfill them and revenue would be pushed into the next quarter.
Given all that non-public information Apple would be able to gauge total sales to within a couple of million units, some of which is probably factored into his revenue range. Tim's revenue estimate is going to be accurate, but I'm also going with the idea it is a little conservative. Nothing Wall Street hates more than missing the target. I predict he has enough knowledge to be confident they will hit the upper end of his estimate or maybe even a little bit more, but not $13 billion more.