Pilot reported inconsistencies with expected approach frequencies within the BOI TRACON airspace that created confusion and is aligned with available published information.

Narrative

I was flying north; returning to ZZZ. A large group of airplanes were to my northeast with a flight path to the north west; so our paths were converging. This particular incident was not a big deal; I have in-cockpit ADSB traffic; so I had plenty of time to descend prior to meeting the intersecting traffic (I passed well below). But because Big Sky Approach has no coherent frequency plans; I didn't think about tuning up Big Sky Approach and seeing if they were talking to the aircraft that I was converging with. For all I knew; they might've been putting them on a descent and my descent would've further complicated the situation. That didn't happen; but it could have. We've got a situation where Big Sky Approach flips frequency sectors around based on active runway at BOI. When I fly near controlled airports; I routinely monitor the appropriate frequency. Why? If I'm causing a problem; I want to know so I can do something different. The SLC sectional instructs 126.9 if north of BOI; and 119.6 if south. Meanwhile; BOI airport data says use 126.9 frequency if within 15 NM; and 119.6 if greater. Neither the chart nor the airport data resembles how Approach assigns area frequencies. You can't look at official information and determine what Big Sky is doing. In meetings available to local pilots; Big Sky Controllers explain how they actually manage frequency sectors; and it has nothing to do with the published information. Rather 126.9 is used for handling finals and 119.6 most everywhere else. This has gone on for years. I get that Boise Approach needs to manage the best way they know how. However; I take objection to a frequency plan that can't be communicated via the chart or an airport data. Either update the information on the chart and in the airport data to reflect how you actually manage the frequency sectors; or come up with a frequency plan that's capable of being communicated through official channels. To their credit; Controllers are usually friendly and professional about getting VFR traffic on the frequency they want you on; but that only fixes the issue for those who a) want to talk to Approach; and b) Approach has to have available bandwidth to set up flight following. This is sloppy management. Please fix it so the information is congruent with practice.