On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.
As soon as you select an analog reference other than EXTERNAL, the
AREF pin gets internally connected to either the AVCC pin (if you select
DEFAULT) or the internal 1.1V reference (if you select INTERNAL).
This typically happens when you call analogRead() or, in your example
code, when you set the REFS0 bit of ADMUX.
Edit: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.