Why does probe request signal varies significantly in the same second?
There are a few factors that can affect wifi signal strength measurements:


Signal Path
Other devices
Distance


If other objects are moving, then this will affect the signal path and the signal strength will vary.

Wifi bands are inherently noisy, a big factor is other devices, especially those that are transmitting in the same band as your device. They are all sending packets, but at different intervals. If you were to look at the wireless spectrum in your area, there could be a lot of noise from other devices.

Another factor is the antenna pattern of each device, which varies according to the spheical angle of the antenna (ie at one point you might get -40dBm, move 5deg and you might get -20dBm). Here is a picture of a 3d antenna pattern from a rasp pi:


Source: https://hackaday.com/2018/04/19/raspberry-pi-w-antenna-analysis-reveals-clever-design/

You should expect the signal strength to vary considerably with any wireless connection, especially those in noisy environments.