An anomaly

[#1]

Reports of average bb speeds were perfectly satisfactory here until yesterday. However ThinkBroadband (which I normally rely on for speed checking) suddenly went bananas, reporting 1.59mb/s on its single D/L check and 9.61mb/s on average over six. U/L report says 14.79mb/s and latency 25ms. ThinkBroadband says the new anomaly may be due to RWIN, which means nothing to me even when reading through the voluminous notes obviously intended for techies.
But BTWholesale check which I decided to carry out at the same time gave 36.37mb/s D/L and 12.88mb/s U/L with 25ms latency. Ookla showed 37.82mb/s D/L with 14.99mb/s UL and 17ms latency on a very local server.Pinging BBC returns 15ms and Plusnet 28ms. These have all been very typical reading on my 40mb/s Plusnet fibre service for ages, testing via ethernet as usual and running under Win8.1. No complaints.
Because of the odd ThinkBroadband readings I ran it three times over the period of a day and rebooted the PC in between, which made no difference. I am not aware of any other changes here apart from the fact that routing morphed from 87.***.***.*** to 91.***.***.*** for no apparent reason. When I rebooted the Plusnet/Technicolor router last night, however, it kindly changed back from 91 to 87 - and ThinkBroadband reported the D/L and U/L readings at the previous (quite acceptable) level.
Should I be concerned? Someone has suggested that there may have been a power glitch while pc was running but I wasn't in attendance? Or is this due to Plusnet suddenly deciding to route my stuff via USofA? Is it likely to recur unpredictably for no apparent reason?

[#2]

So from what I can see, you received 1 odd speedtest results on ThinkBroadband but the other speedtest sites still showed normal speeds?
Don't worry about your IP address changing, this will happen every time you disconnect/reconnect as you are on a dynamic IP.