BT Speedtest not matching Line Speed

[#1]

I have an open (active) ticket in the system relating to my fibre performance, but whilst I wait for the next response I'd be interested in feedback from the people here on the issues to see if the direction I am being taken is right.
Background:
I've had FTTC for a couple of years. BT modem connecting to PN router with a Sam Knows box sitting behind the router and infront of my home network. I'm on the 80/20 Fibre Extra package
Up until recently my line speed has been in the mid 70s but in the last 2 weeks the best I could achieve (all testing done wired) on a BT speed test was 38Mbps up and 16Mbps down. (speedtest.net showed similar test results).
PN Support asked me to connect directly to the BT modem and do a speed test which gave 60.8 Mbps up (max achievable 63.94), 14.5 Mbps down (max ach. 20) and IP Profile of 63.94 Mbps.
PN said the router was at fault and whist I was offered a replacement router free for signing up for another 18 months or £40+P&P I went out and bought a Billion 8800 which combines both modem and router functionality.
I put this in last night. It is reporting line sync of 77000ish down and 19999 up. re-ran BT wholesale test and this has remained in the upper 30s. I thought this may be influenced by the time (early evening) so I re-ran the test at 7am this morning and obtained the same results. Line speed is now being reported by PN as 74.6 Mbps.
The other interesting point is the speed being reported by the Sam Knows box. This is reporting download speed being around 60Mbps but in the last week has dropped to around 57Mbps. Upload speed has been very consistent at around 17Mbps. I'd like to know how it is calculating download speed as it isn't consistent with the BT speed test.
So, it looks like the change of router hasn't brought about the improvements in speed, which points to the router not being faulty. Whilst I wait for PN to reply to my updates on the support ticket would like to see if there are any other suggestions for investigation.

[#2]

Bad form I know to reply to my own ticket but there have been some updates.
BT engineer visit booked for Mon 21 Dec as PN can't find an explanation for why I can't get more than 38Mbps on the BT speed checker but the line is synced at 77,349 kbps. I ran another BT speed test this morning at 6am with everybody in the house asleep and only managed low 20s download.
I've just looked at my Sam Knows stats and as you can see from the attached image, the download speed has jumped from the high 50s to close to 70Mbps which happened 10 days or so after the old BT modem was replaced by the Billion VSDL router.
My question still remains. Why does the Sam Knows box think the download speed is around 70Mbps when the BT Speed test and speedtest.net only report high 30s Mbps download (all testing is wired)?

[#3]

Quote

My question still remains. Why does the Sam Knows box think the download speed is around 70Mbps when the BT Speed test and speedtest.net only report high 30s Mbps download (all testing is wired)?

All a speedtest does is test how long it takes to get a certain amount of data from site X (whereever that is) to device Y (whatever that is). In an ideal set up the result should match the slowest point in the link (your line speed usually) but that needn't be the case. To me it sounds like a fault with the device you're using to test the speed or its LAN connection.
As for inconsistencies with the BT speedtester results that's just the way it is. It has limited value as a diagnostic tool when run from an existing connection. It's slightly more useful if you use the BT login since then you are bypassing your ISP's network. But still it's main value is simply that it ticks a box that BT have on their fault process form.

[#4]

I've run the speed test across at least 3 devices, the last one being the only wired connection to the router (disconnecting the rest of the home network).