Understanding poor WiFi signal quality

TL;DR: How can the wireless signal in Luci be less than 20% if the antennas are literally attached onto the device?

I have a Linksys MR8300 running a recently installed OpenWrt 22.03.2.

Though the speed on my phone is good, I'm wondering how to read this section of the overview page:

The signal strenghts are:

radio0 (5 GHz):    20-52 %
radio1 (2,4 GHz):  70 %
radio2 (5 GHz):    18-50 %

These are approximated values. They appear when I hover the mouse over the signal bars:

Signal Quality

The Maximum transmit power settings are set to driver default, where 20 dBm means 100 mW:

radio0 (5 GHz):    26 dBm
radio1 (2,4 GHz):  20 dBm
radio2 (5 GHz):    23 dBm

I also use a WiFi Analyzer on my phone. I can tell you my WiFi is NOT the best signal I receive, except for when I'm standing very close to the router.

Somewhere here in the forum I read something like this regarding the MR8300: Before booting/installing OpenWrt, one should boot the original firmware, enable WiFi and wait for a half our, in order for the radio to adjust something with DFS (Dynamic Frequency Selection). Something like that. I followed that suggestion.

My question is: How can the signal be even less than 99% if the antennas are literally attached onto the device?

The host I'm logged in into Luci has a wired connection only. Having only one 5 GHz radio enabled, doesn't improve its quality.

Here are my wireless settings:

=> uci show wireless
wireless.radio0=wifi-device
wireless.radio0.type='mac80211'
wireless.radio0.path='soc/40000000.pci/pci0000:00/0000:00:00.0/0000:01:00.0'
wireless.radio0.band='5g'
wireless.radio0.htmode='VHT40'
wireless.radio0.channel='140'
wireless.radio0.country='DE'
wireless.radio0.cell_density='0'

wireless.default_radio0=wifi-iface
wireless.default_radio0.device='radio0'
wireless.default_radio0.network='lan'
wireless.default_radio0.mode='ap'
wireless.default_radio0.encryption='psk2'
wireless.default_radio0.ssid='MyWLAN5'

wireless.radio1=wifi-device
wireless.radio1.type='mac80211'
wireless.radio1.path='platform/soc/a000000.wifi'
wireless.radio1.band='2g'
wireless.radio1.htmode='HT20'
wireless.radio1.channel='10'
wireless.radio1.country='DE'
wireless.radio1.cell_density='0'

wireless.default_radio1=wifi-iface
wireless.default_radio1.device='radio1'
wireless.default_radio1.network='lan'
wireless.default_radio1.mode='ap'
wireless.default_radio1.encryption='psk2'
wireless.default_radio1.ssid='MyWLAN'

wireless.radio2=wifi-device
wireless.radio2.type='mac80211'
wireless.radio2.path='platform/soc/a800000.wifi'
wireless.radio2.channel='36'
wireless.radio2.band='5g'
wireless.radio2.htmode='VHT40'
wireless.radio2.country='DE'
wireless.radio2.cell_density='0'

wireless.default_radio2=wifi-iface
wireless.default_radio2.device='radio2'
wireless.default_radio2.network='lan'
wireless.default_radio2.mode='ap'
wireless.default_radio2.encryption='psk2'
wireless.default_radio2.ssid='MyWLAN5'

omitted lines:
wireless.default_radioX.key='***'