The house could have been open/closed. Weather can cause changes. There are many reasons for the changes. That's why they do an average. Your inspector can tell you more.

https://www.doctorhomeair.com/changinglevels/

All the readings are below the action level and the average is well below the action level. The variability in the readings is normal. Radon reduction systems are pretty cheap, so put one if if you want to drive the numbers down further, but that graph doesn't look very alarming to me.

The US has the lowest radon level of any developed country. The allowable amount is much higher in Europe. Anyhow, don't be concerned about your test results. They are well below acceptable ranges within the US and they naturally fluctuate during the test. All is well.

Did you have doors or windows opening and closing? You can always repeat the test to see where your levels are.

If your windows were open, you can redo the test in the winter. We had our house tested in winter and it needed radon remediation (about half of the houses in the country need remediation)