IMO the BBMax speedtest doesn't transfer enough data for a reliable test on anything above ADSL speeds

I was shocked earlier on. On BBMax I often get 67Mb for my 60Mb connection. But tonight I got 75Mb! How am I getting that speed when it's 25% more than I'm supposed to get, and is it accurate? BBMAx is always the one that gives me the highest speeds. Meanwhile, I'm getting differing speeds elsewhere. TB giving me 57Mb java and 55Mb flash. Speed test.net gives me 62Mb on average but is currently fluctuating between 56Mb and 62Mb.
http://www.speedtester.bt.com is giving me a consistent 66 to 67Mb. I've no idea why they all vary so much but I wonder what I'm really getting and which is the true speed tester.
I have another question. VM engineer came to my house on saturday and fitted a 10 dB forward path attenuator as my power levels were over 11 dBmV and are now down to 0. I asked him the benefit of this and he said it would ensure my download speeds never go below 60Mb. Well apparently not as I'm still seeing sub 60's, albeit not very much. So, is he talking rubbish?