The short answer is that the field capacity is the difference in moisture level between total saturation and the allowed depletion percentage. So increasing the allowed depletion means more moisture can be lost before watering is required.
I notice that changing the units from US to metric also changes the Available Water value for the zone. I am guessing that the number shown in metric units is the available water in CM per inch of root depth. It would have been simpler if Rachio made the AW a percentage - the amount of water that each unit of root depth can hold. In US units, multiplying the AW x Root Depth gives the total water available for the root zone, given the soil type. Multiply that number by the allowed depletion to get Field capacity.
Looks like you need to divide the AW by 2.5 to get the amount of available water per cm of root zone. So if you use .2 for AW instead of 0.5, your calculation is AW (0.2) x RD (15 cm) x AD (0.2) = 0.6 cm in the first case. If you use 80% allowed depletion, you can lose 4 times the water (2.4 cm) before watering is needed.
I would prefer if the moisture level graph tracked the absolute moisture level - where 100% is saturated, and 0% is absolutely no moisture. The allowed depletion level can be a dotted line to show when watering is needed. Changing the allowed depletion would simply move the dotted line on the graph, not change any numbers. Maybe in V3!