Why do some routers use non standard 2.4ghz channels when set to auto?

I work for a WISP, and we've came into many areas where other providers use routers set to "auto" meaning they'll use channels 2,3,5,7,9 religiously for the most part. We usually set our gear to 1,6 or 11. When we have issues, which we've had (we've seen speeds drop to 1mbps due to interference) , we try to talk to the neighbors and set up their equipment on 1 6 or 11. Not always possible

I'm wondering why some mainstream gear such as TP Link tends to choose non- "interference free" channels. Not all brands do it, but the most common brands we come in contact with absolutely do.