then you'd almost never be able to compare anything in the car world.
but it is possible to look at the data and make sensible inferences.
the facelift 350 had more hp than the pre-facelift, so you could reasonably expect it to be faster than the pre-facelift, and it was.
the r171 55 is heavier than the 350 - affecting braking and handling, and it's always going to be hampered by the auto transmission vs the manual in the 350. combine that with a track that doesn't let the 55 use its power to best effect and the result isn't really all that surprising.
the r172 55 has more hp and some other trickery (like the inside rear wheel braking in cornering), so it's reasonable to expect that to be faster than the r171 55 - and it was.
does that sound reasonable to you - or do you think there's something specific that driver/conditions have skewed?
Your argument is like saying A beats B, C beats B, thus C beats A. I didn't say it is impossible that is the case, but you certainly haven't proven it so.
The driver is one of the most important features in any track time.
We know nothing about who drove the cars - there's almost 3 whole years between the two runs. If we can at least say they are both high-level professional drivers, that would help, but who knows??
The SLK55 run was in June, whereas the facelift 350 run was in April, which likely helps since it is cooler but not cold enough for the track surface to be negatively affected.
You also left off the difference in brakes, which is also very important.
You can't just say one car is slightly heavier but has an automatic (with a manual mode) so that offsets the much better brakes and 50 more horsepower. Did the SLK55 in either run use manual mode? I didn't look at the facelift article, but there was certainly nothing in the initial article about it.
I'm saying there are too many differences and unknowns to make a direct comparison between the two.