Speed explaination

Could someone please explain why this happens?

Date Down HTTPx6 Down HTTPx1 Upstream IP Address ISP
Sun 06/06/2021 18:49 0.54 Mbps 0.33 Mbps 5.59 Mbps xxx.xx.xx.xxx Plusnet
Sun 06/06/2021 11:43 71.36 Mbps 67.72 Mbps 18.35 Mbps xxx.xx.xx.xxx Plusnet

Two vastly different results less than 7 hours apart?

No idea why that happens but similar thing on my line from 2mb down to 49mb in few minutes.
Also vast difference between which BB test and other sites.