Look at this thread (XKnight ran into the same issue): viewtopic.php?f=6&t=28946FunMaker wrote:Did not find an anwser through searching - maybe someone knows:
How do stack filter inserters set the filter through circuit network if multiple values are present on the circuit network?
Hint: It's not the highest value on the Network that is taken for the filter.
Seems a bit random to me
The signal that is chosen is basically undefined. It's probably due to some arbitrary ordering of the signals in the Factorio implementation (maybe related to the order of the signals in the UI, maybe not). XKnight suggests it might be some "signal ID" which is not visible to the player).
If you want to narrow down a bunch of signals to a single one, you're in a world of pain (it's not easy). The enhancement to the decider combinator that XKnight proposes in that thread is to have an easier solution for this problem; it would help with setting the filters and also with a host of other unrelated problems.