My deco s7 show fair signal strength with -40dbm to -45dbm.

On my deco app and not full bar.. why is this going on is there and app issue. On when s7 signal strength -35dbm its show full bar and strong signal strength. Is there a bug in app or on s7 deco.

On my other deco e4 router even -48dbm shows full bar and signal strength strong.