Zeiss always seems to have an edge over Leitz and all the other lens manufacturers of the period. Overal Sharpness-wise I mean.
I guess the soft (crown?) glass of the front elements of the Summars and Summitars and 70 years of grime collecting on the internal surfaces all add their bit to the 'reputation' of these lenses. Though I've seen fine images from scratched samples. Certainly when mounted on modern digital bodies.
And there's the fact that a lot of the younger lenses are, simply, just better.
The front and back of my Xenon have plenty of cleaning marks and it has quite a bit of glow in high contrast situations with all those surfaces the light has to pass through. A hood helps.