8400 S-meter readings strange behavior

RX3APM

Hi to all,
My new 8400 is finally arrived.
First of all I try to compare with two 6400 I have.
I note difference in S-meter readings.
On the same frequency, same signal (or even without signal), same bandwidth, same preamplifier settings 8400 shows about 12db (two S-units) more than both of 6400.
This goes against the IARU standard scale requirements as of -73 dBm for S9 and -141 dBm for S1.
Both 6400 S-meters shows around -140dBm for 400hz bandwidth, +32db preamp on 28mhz band without antenna connected.
8400 with the same setting shows just -128dbm.
When the antenna is connected and the signal is present the difference remains let’s say S7 on 6400 and S9 on 8400.
I checked this with two brand new 8400, mine and one of my friend.
Any ideas what’s going on and how to fix it ?

73, Sergey RX3APM