Sounds like you’re hitting the 40/20MHz item in 2.4GHz - OpenWRT complies with 802.11n, where the primary channel is 20MHz, and 40MHz is opportunistic.

Don’t know how MT and Mango deal with this, but AR’s do 20/40, and I suspect that MT does this as well, as this is standard behavior

2 streams in 802.11n at 20MHz maxes out at 150Mbps