I have recently upgraded from ADSL+ (17Mb down 1Mb up) so have been reviewing the speed tests available. My Infinity 2 sync is 80/20 (I am 60m from the cabinet). Funny BT wholesale said my thruput was 8MB! and upload was more. I have found this and most speed check services using flash or java seem now to be unreliable. Whilst I expect actual thruput to be lower than sync speed due to overheads I think BT ought to redesign their tester which used to work very well. I have found the most useful one to be 'speed of me' (speedof.me) which says it's purely html5.
This reports my actual thruput of 80.89/19.43 with latency of 70mS from a London server, which seems more realistic although I realise my circuit will be 'training' for 10 days. Also it also took 3 days for my BTFON wifi to turn off on the new home hub 6 router (BTFON seems to be default to on, on new routers) which also looks good to me.
So really do not dispare if you see wild fluctuations in speed testers, quite often they are inaccurate.
Btspeedtester has been giving bad results in some cases especially if using Firefox - chrome and Edge/IE appear ok
the speeds of 80.89/19.43 must be your connection speeds not download speeds