By default FX always pass audio to the outer context. The sound_out* FX are no exception. What they actually do is both route the audio to the specified sound card output and route the audio to be outer lexical context. In your case these end up in the same audio card output audio channels hence the confusion.
If you were to output to audio card channel 10, the audio will be heard both on 10 and on 1,2 as expected as it’s the outer FX.
To only route the audio to channel 10, you would need to supply an amp: 0 as an opt to the :sound_out FX to suppress the standard lexical output behaviour but only after the FX has routed to channel 10.
Hope this makes sense 