First you should download a proper speed testing software as the 3BB test is not reliable, the technician come to my house as I had the same/similar speed inconsistencies as the tech didn't use their own testing software. I use Analti.
Second, speed varies mostly depending where your routing/modem is trying to connect or it's connected, if nearby or faraway server, I found out (at least for me) my router/modem doesn't connect to the same server at all times and thus I have a big variation of speed
Third, if you see a drop in speed or inconsistency in speed, easy fixing is to re-start the modem, I have to do it 2-3 times a week to maintain a steady average speed
I have a 1 GB/300 from 3BB