I believe the different trip currents are mainly due to the common breaker panel RCD protecting many circuits, compared to one integrated into an outlet. (Those do exist here btw, even though they're quite rare.)
When a single device is allowed to have a current flowing through PE up to 3.5 mA (greetings to filters in PSUs) a for example 5 mA limit for an entire appartment worth of circuits would not be very sensible. Of course the currents are usually lower. But actual RCD trip currents also tend to be a bit below the specified current.
The 3.5 mA are the limit for PE current a device with a grounded plug is allowed to have in regular checks of appliances. These checks are mandatory in public and commercial environments in Germany btw, depending on the environment every three months (construction sites) up to two years (offices) for everything mains voltage with a plug.