Because there is no internal voltage reference.
Keep on reading, the table for VCFG values shows that you can use AVDD/VREF+ for positive reference and AVSS/VREF for negative reference, in different combinations.
Then, as it's mentioned at the beginning of the datasheet
70000689d.pdfUp to Four Op Amp/Comparators with Direct Connection to the ADC module:
- Additional dedicated comparator
- Programmable references with 32 voltage points
- Programmable blanking and filtering
the voltage reference with 32 levels is for the comparator only (CVREF) and, as the OP AMP/COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM will tell you, CVREF is derivated from either AVDD or VREF+.
if you want an external voltage reference you'll have to use VREF+ (No need to use VREF-)
otherwise, fix your board layout
-a small resistor,inductor or ferrite bead (chosen with some criteria) between the main VDD and AVDD and decoupling caps between AVDD/AVSS.
-small capacitors next to the analog inputs to decouple the signal (that will reduce the bandwidth of the signal)