Why are you getting a Speedtest for BT Infinity when you are on PLusnet? Were you on someone else's wireless at the time?
Probably because BT have moved some IP address ranges over to Plusnet, and Speedtest.net hasn't updated - but Thinkbroadband has.
IPv4 blocks are moving all the time as they have run out - expect to see more of this until we all get IPv6.
plusnet unlimited fibre
80/20 since 2 Jun 14 - Sync as of 5th Feb 17: 57,821/9,173 kbps with G.INP
18 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM