asterisk users - May 2014 - dahdi-dahdi native bridging and audio level

Hello!

I use asterisk with TE420 as PRI switch for two channels :

;panasonic uplink
         group=3
         context=panasuplink
;        relaxdtmf=yes
;        immediate=yes
         rxgain=0.0
         txgain=0.0
         mohsuggest=default
         jbenable = no
;        jbenable = yes
;        jbmaxsize = 200
;        display_send=name_initial
         display_send=name
         display_receive=name
;       display_receive         channel=>63-77,79-93

;panasonic
         group=4
         priindication = outofband
         rxwink=300
         pridialplan=unknown
         prilocaldialplan=unknown
         context=panas
         rxgain=0.0
         txgain=5.0
;        signalling=pri_net
         switchtype=qsig
         overlapdial=yes
         resetinterval=never
         immediate=no
         mohsuggest=default
         jbenable = no
;        jbenable = yes
;        jbmaxsize = 200
;        display_send=name_initial
         display_send=name
         display_receive=name
;       display_receive         channel=>94-108,110-124

Initially I had no txgain=5.0, but users complains that  they hear not 
as good as before asterisk was added into this scheme.
We even tested and found that yes, asterisk decreases volume.

Could you tell me why? As I understand this is digital connection and 
there is the same alaw codec, so no trancoding.
Why can be sound volume changed during chan_dahdi native bridging?

Thank you!