Root causes + solutions:
1. Regulatory data might be outdated or wrong
Fix regulatory bugs and report them to OpenWrt and/or upstream at: http://lists.infradead.org/pipermail/wireless-regdb
iw reg get
2. Hardware is not constructed or does not support high transmission power
2.a Hardware design (target market) plays a role
High tx power requires different hardware design (more costs not worth on consumer hardware). Most use cases do not need high tx power but can profit from better antenna or multiple APs.
2.b Hardware limits
see Background-1 & MCS ratings w. tx power on data sheets (example: WLM200N-23ESD). Setting higher limits can damage or shorten the lifespan of the hardware. Hardware quality varies in production process and individual calibration is costly. Use high tx power cards if you need them or can operate them in your regulatory domain. Some regulators might allow operation for radio amateur or via some license scheme.
3. Driver/Implementation limitations
The driver does not use some circuitry (on RX path: LNA, example) Factory firmware, OpenWrt and some vendor firmware based on OpenWrt could use different drivers. Each driver can interpret limits differently. Without measuring equipment the output power level could be reported wrong. Power management can influence neighbour APs (noise floor,hidden station).
4. Firmware/ART/caldata/EEPROM/OTP limitations
“Firmware” means the blobs loaded by fullmac devices (many 802.11ac chipsets) can introduce non changeable tx power or different tx power depending on driver.
“ART/caldata” means a dedicated partition on system flash containing data to setup ath9k (ART=atheros radio test) or other hardware (caldata = calibration data).
"EEPROM" means a dedicated chip contains the setup data (common on older devices).
"OTP" means the data for the setup is inside the main wlan chip.
Many countries enforce conformity tests (FCC, CE…) to ensure that the hardware operates within reasonable limits. Doing these conformity tests is often not done on very cheap hardware or the hardware is programmed and designed to operate at save values instead of operating at the “regulation border” at max. allowed txpower. Some regulation infos can be part of the ART/caldata/EEPROM/OTP.
Some devices contain multiple ARTs. Using high power tables can damage the device - see this commit