From the Tiny45 RevG datasheet (still unlocked):
• Bit 7:6,4 – REFS2:REFS0: Voltage Reference Selection Bits
These bits select the voltage reference (VREF) for the ADC, as shown in Table 20-2. If these bits are changed during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSR is set). Whenever these bits are changed, the next conversion will take 25 ADC clock cycles. If active channels are used, using VCC or an external AREF higher than (VCC - 1V) as a voltage reference is not recommended, as this will affect the ADC accuracy.
Any guesses on what an "active channel" is? Fast-changing signal?