That adaptation value does not tell you what is causing your lean mixture.
On the W124 HFM-ECM, the adaptation limit is +- 25%. So, at 1.26, the HFM module would store an Adaptation at limit fault code.
The Engine Control Module tries to keep the air/fuel mixture at the optimum level. There are numerous reasons why at times it is unable to do so, eg. Fuel injectors, vacuum leaks, electrical problems, modules etc.
Ignition Analyzer scope can give you an insight into what's going on at each cylinder, also check the adaptation values at different engine speeds. If the adaptation values are worst at idle, you probably have a vacuum problem. If it is a fuel related problem, it will probably get worse at higher engine speeds.
I suggest getting alldatadiy and familiarize yourself with the various systems of your car. Then develop theories. Based on these theories, you can design experiments and checks to collect data. Then use this data to investigate further in the direction you feel the data is pointing you, and it may lead you the cause of the problem. Sometimes thou, you may have to start eliminating various causes, including replacing parts with good known parts to see if that fixes the problem.
Hope that helps.
94 E320 58000 Miles