RSSI by antenna

Hello,

I use Openwrt 21.02.3 on a Linksys EA8300.
I'm connected to a 5GHz SSID and I want to check the RSSI of each antenna. For this, I use the command iw :

iw dev wlan2 station get '30:B1:B5:D4:40:54'
Station 30:b1:b5:d4:40:54 (on wlan2)
        inactive time:  1800 ms
        rx bytes:       27296708
        rx packets:     133915
        tx bytes:       20749
        tx packets:     258
        tx retries:     0
        tx failed:      0
        beacon loss:    0
        beacon rx:      63953
        rx drop misc:   21
        **signal:         -45 [-52, -53, -99, -99] dBm**
        **signal avg:     -48 [-50, -51, -99, -99] dBm**

If we check the line : signal: -45 [-52, -53, -99, -99] dBm
Signal should be -45dB, but what are the numbers between [] ? RSSI by antenna ? And I wonder why the last two numbers are so low : -99.

Thank you.

Christophe.