Assigning power manually didn't help.
Well, I've finaly found where the strange power levels come from - "show log driver":
Mon May 20 15:47:47 2013:55 10.223106 wifi0: EIRP (Config: 18.0dBm, Set: 13.0dBm, AP Max: 20.5dBm, Reg Max: 20dBm), Conducted Power Set: 10.5dBm, Gain: 2.5dBm
So I did quick comparing to IAP-105 (now both APs are 3.3.0.x) and guess what - it is different:
(ARM set on both APs to min/max 18/18db)
IAP-105 :
Mon May 20 15:34:19 2013:155 148.824448 wifi1: Country Code Set to CZ
Mon May 20 15:34:19 2013:156 148.845601 wifi1:(3) EIRP (Config: 18.0dBm, Set: 7.5dBm, AP Max: 22.5dBm, Reg Max: 20dBm), Conducted Power Set: 5.0dBm, Gain: 2.5dBm
Mon May 20 15:34:19 2013:157 148.845682 wifi1: Channel Set to 11 (mesh-mode: 0, state: 5)
Mon May 20 15:34:19 2013:158 148.845728 wifi1: Internal Antenna Gain Set to 2.5 dBi for 2.4 GHz
Mon May 20 15:34:19 2013:159 148.845771 wifi1: Internal Antenna Selected
Mon May 20 15:34:19 2013:160 148.845810 wifi1: disabling 40Mhz
Mon May 20 15:34:19 2013:161 148.845861 wifi1: RADIO CONFIG DONE
Mon May 20 15:34:19 2013:162 148.845898 FREE MEMORY 71232(kB)
Mon May 20 15:34:19 2013:163 149.040352 Refetch EIRP
Mon May 20 15:34:19 2013:164 149.040405 wifi1:(5) EIRP (Config: 18.0dBm, Set: 18.0dBm, AP Max: 22.5dBm, Reg Max: 20dBm), Conducted Power Set: 15.5dBm, Gain: 2.5dBm
*** Real signal reading with INSSIDER around -25dB ***
*************
Compare to :IAP-92 + AP-ANT1( 3db/5db antenae, set in GUI)
Mon May 20 15:47:47 2013:50 9.826019 wifi0: Effective External Antenna Gain Set to 2.5 dBi (connecter loss 0.5) for 2.4 GHz (Configured Value: 3.0 dBi)
Mon May 20 15:47:47 2013:51 10.201628 wifi0: Country Code Set to CZ
Mon May 20 15:47:47 2013:52 10.222664 wifi0: EIRP (Config: 63.5dBm, Set: 13.0dBm, AP Max: 20.5dBm, Reg Max: 20dBm), Conducted Power Set: 10.5dBm, Gain: 2.5dBm
Mon May 20 15:47:47 2013:53 10.222796 wifi0: Channel Set to 6 (mesh-mode: 0, state: 5)
Mon May 20 15:47:47 2013:54 10.222869 wifi0: External Antenna Selected
Mon May 20 15:47:47 2013:55 10.223106 wifi0: EIRP (Config: 18.0dBm, Set: 13.0dBm, AP Max: 20.5dBm, Reg Max: 20dBm), Conducted Power Set: 10.5dBm, Gain: 2.5dBm
Mon
**** Signal reading in INSSIDER: -40 dB at same AP possition ****
***************
Seem's that:
1) we have a REAL problem here, signal measured on PC is lower
2) AP92 driver code for some reason subtracts 5db (sometimes less)