I note that:
1) the specifications for the Teeny 3.2 state it has a ADC resolution of 16 bits in hardware and useable resolution on 13 bits
2) the specifications for the Teeny LC state it has a ADC resolution of 16 bits in hardware and useable resolution on 12 bits

I am told the reason for the lower "useable resolution" is noise in the least significant few bits.

"ADC resolution commonly found on-board microcontrollers, such as the Teensy 3.2, have a full scale resolution of 12 to 16 bits, with usable resolution in the range of 10 to 13 bits. The difference between full scale resolution and usable resolution is noise in the least significant few bits. The actual signal measurement range of an ADC is a function of usable resolution and of the ADC reference voltage. A 13-bit usable ADC with 3.3V reference therefore has a resolution of 3.3V/213 or 412.5μV"
Source: AN IMPLEMENTATION OF A HIGH ACCURACY SENSOR FOR DISTRIBUTED WIRELESS SENSOR NETWORK APPLICATIONS by JESSE CRAIG FRYE

Three questions:
1) Is there really a difference in the "useable resolution" between the 3.2 and the LC ?
2) Has this difference been proven/demonstrated in testing and evaluation ?
3) How does the "useable resolution" compare to an external 16 Bit ADC such as the Adafruit ADS1115 ?

and yes one "silly little bit" does make a difference.

Thank you