I follow what you are saying. To be honest I cannot give you a specific answer to why each leather is reacting different, but I can generalize it.
Think if how you can get three different results on three different types of paint. Each paint can be the same (in system and concept), but completely different in how they are manufactured and applied.
Leather in the 3-4 cars you list is completely different in the way it was grown (cow and breed - although most leather comes from slaughter and dairy cows), harvested, tanned, dried, polished, and sealed (if it is coated). When it comes to cleaning (think washing a car), they will all come clean the same way and you will see a dramatic difference in both look and feel - you are just removing dirt and grime. The conditioning phases are where you will get differing results (think correcting and waxing) with the same product. The age, condition, grain, tan of the leather will really change how your product is reacting and working.
I have some Conolly conditioner, some cheaper MB leather seats it is absolutely wonderful on, my moms sofa - YIKES!
My recommendation, have an assortment of conditioners and test on small test spots to see what they do, then choose from there.
For gentle cream conditioners these are great:
- Raceglaze
- Zaino Leather
- Conolly
- Black Label Leather Conditioner (haven't tried this yet, but seems nice)
For more aggressive conditioner.
- Leatherique
- Polish Angel Charisma
- SV Bees Wax