ADC measuring internal reference gives strange results for shorter sample times. Is there a way to measure the internal reference with sample times of 1.5?

[CVan.18]

Setup: 48MHz CPU clock, ADC clock = CPU/4. For various sample times, I get internal reference measurements of: 1.5 - 1925, 7.5 - 1760, 13.5 - 1420, all other sample times give 1550. I thought sample times were to cope with high impedance sources. Is there also the need for long sample times when measuring the internal reference?