why does the AS726x library
Ask the author of the library.
The device data sheets are your best source of advice. For the AS7263, gain = 16x is used in the specifications summarized in Figure 12 of the data sheet, and results in the given typical output sensitivity.
If you want to use this device quantitatively, you will have to calibrate each one using known intensity/wavelength standards, using any of the several possible gain settings.