As already suggested, the one premise that fits the calculations is that cities start with 1 citizen, but that citizen works the city tile rather than any surrounding tiles (which you probably won't have access to until tile growth occurs anyway). Further tiles as they become available can be worked by citizens 2 and up.
Arguably this makes more sense than the process of having an extra worked tile at population 1. We are just used to the city tile being a "free" resource in Civ until now.
Furthermore, and reinforcing the "first citizen as city tile only" is that when you add up all the food in the city screenshot, there is 21 food (including city tile), and the overall nett food is +1 for the whole city. Assuming citizens still consume 2 food and there are no health subtractions we can't see in the screenshot, 21 food is good for only 10 citizens with 1 food left over. This suggests that the first citizen doesn't consume food.
So the city in the screenshot probably started off with one citizen working the city tile, no non-city tiles and +2 food allowing for eventual growth in the population.