Why do NOS Tubes sound better than new tubes?

Agree with Tpreaves. I've replaced Mullard xf1 EL34s as driver tubes in my SET monoblocks with current-production Genalex Gold Lion KT77s because they sound better. OTOH, NOS tubes are often preferable. But it's not a sure thing and always worth trying the new ones.

I heard a story about an RCA tube making machine (whatever they are called) that was new in the 20s and made great tubes but in the 30s the fittings got looser and the 40s looser yet etc. etc., each year tube quality went down due to worse tolerances. They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.

One interesting explanation...

They kept the machine for several decades so tube quality at the end of the machines life was relatively poor.

Could explain why there are so many complaints about new records people have to keep returning.

A lot of NOS tubes sound bad. There are good ones and bad ones. I prefer the Winged C 6550 to the GE 6550 for many reasons. First, the WC sound better, less shrill treble, better bass, and more reliable. I do like the Amperex BB 12AX7's. But I hear from friends, they have replaced theirs with the GL 12AX7's and like them as well or better. I think new tubes can be very good and just be open minded before spending 300.00 for a rare NOS that may not be any better.
Tpreaves is asbsolutely right.

Better trained people and better materials back then. As far as one tube sounding better than another, it depends on the application.