Surely you really cant compare the two at all. For a start, different days, so different temperatures, different weather conditions, potentially different road surfaces (even if tested at the same location) etc...
Then you have different tyre sizes.
Then you have different cars. I mean, who would have thought the heavier 5 series might be more difficult to stop than the lighter 3 series.
Different starting speeds too.
Potentially different drivers.
Basically, nothing about these tests are consistent enough to compare them against each other. They are no doubt consistent enough to compare tyre performance within in each test. But certainly not against each other.
The only way you can realistically "measure" the improvement of tyre technology over time, is to accumulate percentage increases when new and old models of tyres are tested back to back on release of the newer tyre. Then when that tyre is tested against it's predecessor and so forth. Very rarely is the new tyre outperformed by the old tyre. And so adding up the accumulated improvements per generation might be the only real way to get any meaningful evidence of improvement.
As has been said before, tyres degrade over time, regardless of use, and so finding a "mint" set of well preserved 20 year old tyres and comparing them to current tyres simply wont work. However these 20 year old tyres were likely tested against their predecessor, and it against its own predecessor, until you get to the current tyre. And so the accumulated improvements over each test would be the only real indication. So adding up improvements from PS1 to PS2 to PS3 to PS4 to get the improvement from PS1 to PS4.
Of course, you can go beyond that too. Tyre technology has improved such that there are now "higher performance models" on the market. So do you compare the original Goodyear Eagle F1 to the Asy 5? Or the SuperSport / Supersport R / Supersport RS? I mean, they all represent the "ultimate" road tyre of their time, to some extent at least.