The internal 1.1v reference on the AVRs is not quite super accurate either. It's good enough in most cases like displaying a battery voltage.
To get accuracy you need some external dedicated reference.
The SAMD21 also has some internal references, I have not spent too much time with these but I expect the accuracy to be similar to the AVR or perhaps better.
These are the REFSEL options for the ADC reference:
0x0 INT1V 1.0V voltage reference
0x1 INTVCC0 1/1.48 VDDANA
0x2 INTVCC1 1/2 VDDANA (only for VDDANA > 2.0V)
0x3 VREFA External reference
0x4 VREFB External reference