Stereophile's 2021 products of the year

I have no opinion as I have not read the review, however it may be sample variation, as you have surmised, because Stereo Magazine found that it measured well

https://stereo-magazine.com/archive/articles?tx_archive_pi1%5Baction%5D=download&tx_archive_pi1%5Barticle%5D=69341&tx_archive_pi1%5Bcontroller%5D=Archive&cHash=b63605b6cab9c40ddc6dbc5e2d1dd3ed

‘Low distortion, low noise, high bandwidth and
an enormous attenuation factor for a tube amplifier.’

Hope this helps.

Did they measure-the same one they reviewed?

It is interesting that the stereo-magazine.com article does not indicate the distortion level at 64/65 watt continuous output as well as other problems Atkinson found.   https://www.stereophile.com/content/vac-sigma-170i-iq-integrated-amplifier-measurements  However, on-line I just read multiple explanations that point to a need to carefully select a partnering speaker, preferably above 90db efficiency and easier impedance phase angles.  This is rated as an 85 watt amp but reaches 3% THD on a slow rise at 51 watts, 2nd harmonic distortion type which may not be hearable versus a similar solid state amplifier with the same distortion but with a quick rise.  Herb Reichert, Jim Austin and John Atkinson further explain the measurements.   They were answering @George who stated from the review  "Using our definition of clipping, which is when the output's THD+noise percentage reaches 1%, the amplifier with both channels driven with a 1kHz signal clipped at 11.8Wpc into 8 ohms."   I can't say George doesn't have a good argument that the amp section is very particular of it's speaker mate with a low damping factor and high impedance resulting is potentially great frequency response variations.

I think Jim Austin gave a pretty good account of the design intent: ie deliberately eschewing feedback in the interests of improved SQ into tube friendly speakers - rather than designing a panacea for all speakers. It‘s also interesting to note that Jim confirms that an amp that measures worse (ie without feedback) can actually sound better.

He also made the interesting point that some speakers like to see high output impedances. From memory, I think these these mainly fall into the single driver category. Speaker matching is always important to a certain degree.

@fleschler  I agree entirely regarding sensitivity and impedance/phase angle effects on the in room efficiency of a speaker. I have read many speaker reviews that don't adequately address this. 

I agree entirely regarding sensitivity and impedance/phase angle

This is why I always try to show Stereophile speaker impedance vs -phase angle graphs when recommending an amp for anyone.
Instead of just blindly saying get this amp it's the best.😵

Cheers George