I had this exact question at one time. Having owned a manual transmission Subaru, I can tell you what my cousin co owner of a Subaru dealership told me was the thought behind theses two different awd systems with different transmissions.
With a manual, generally the buyer is more apt to prefer the handling that rear wheel bias provides. Hence, up to 100% to rear wheels. Automatic owners are less inclined to care or even want the rear end kicking out, hense the limit at 50%. This is because automatics are bought by less aggressive drivers…in general, who do not care about this handling preference of rwd bias.
So, it’s an effort to match the transmisionn with the suitable awd torque distribution bias. Car makers do this all the time with packaging options together for example either, luxury or sport or economy lines with different size motors, to transmissions, tire size, final drive ratios etc. It’s with the intention of matching all the diferent drive train components to different perspective buyer preferences. It’s obviously somewhat of a guessing game but with the use of statistics. He who does it best, sells the most cars to the most “different” buyers in the same model line.
Let me say I loved the handling of our manual trans Subaru vs the auto of a friend. My wife liked it so much she ruined a set of tires driving it too agressivly when new.