How is the lower voltage on a transformer calculated?
That lower voltage value is listed under the general heading "target application" and I'm sure if you dug deeper you'd find that target application and be able to show a circuit diagram then all would become obvious.

It also says (under that target application) that the output voltage is 5V yet the LT circuit shows 15 volts so you need to dig a bit to find that circuit.

The LT circuit is a flyback converter and the ratio of DC input to DC output voltage has very little to do with turns ratio and much more to do with duty cycle. For instance, the transformer (aka coupled inductor) in question is ratio 1:1 so, as far as duty cycle is concerned I'd expect input voltage to equal output voltage at 50:50 (this is a broad brush impressionist type remark of course) and if you do the sums (below) it is not that far off....

Input voltage range is 2.7 volts to 36 volts and this averages about 19 volts so having 15 volts at the output is "about" right (generalisms used with caution of course) for the mid range of input DC voltage.

Like I said earlier, it's a flyback circuit and duty cycle is king and basically turns ratio is the parlour maid: -



She's the one at the back on the left and he's the one at the front calculating the duty cycle.