Why does BT speed test show persistant lower speeds?

[#1]

Can anyone tell me why the BT speed test keeps giving me low download and upload speed results going between 15 Mbps and 23Mbps. I have a piece of software called Netmonitor that monitors the upload and download speeds. When the BT downoald test is running it shows  between 36 and 38 Mbps.
When ever I run the speedtest.net test the netmonitor shows the same download and upload speeds as were there during the BT test but the actual speedtest.net results give me between the 36 and 38 Mbps for download and about twice the upload speed than the BT test. I have tried other speed tests and they all give better speeds than the BT test.
These tests have been tried on three different machines so it is not a software or hardeware problem.
Gerald

[#2]

Are you testing via a wired Ethernet connection?

[#3]

Yes, wired on all three machines,  either only one connected, or two, or all three.

[#4]

Would you mind posting the results from the Further Diagnostics page which should ask you for your landline number and give you your IP profile.

[#5]

1. Best Effort Test: -provides background information.
Download Speed
20.81 Mbps

0 Mbps 37.05 Mbps
Max Achievable Speed
Download speedachieved during the test was - 20.81 Mbps
For your connection, the acceptable range of speedsis 29.64 Mbps-37.05 Mbps .
Additional Information:
IP Profile for your line is - 37.05 Mbps
2. Upstream Test: -provides background information.
Upload Speed
2.96 Mbps

0 Mbps 20 Mbps
Max Achievable Speed
Upload speed achieved during the test was - 2.96Mbps
Additional Information:
Upstream Rate IP profile on your line is - 20 Mbps

This test was not conclusive and further testing is required.This might be useful for your Broadband Service Provider to investigate the fault.
If you wish to carry out further tests,please click on 'Continue' button.If not, please close the window using 'Exit' button and contact your ISP for further assistance with these results.

[#6]

Tested again 23:30 Tuesdat 18th August 2015, with these results

FAQ
Results Image not loaded

1. Best Effort Test: -provides background information.
Download Speed
19.78 Mbps

0 Mbps 32.4 Mbps
Max Achievable Speed
Download speedachieved during the test was - 19.78 Mbps
For your connection, the acceptable range of speedsis 25.92 Mbps-32.4 Mbps .
Additional Information:
IP Profile for your line is - 32.4 Mbps
2. Upstream Test: -provides background information.
Upload Speed
3.21 Mbps

0 Mbps 20 Mbps
Max Achievable Speed
Upload speed achieved during the test was - 3.21Mbps
Additional Information:
Upstream Rate IP profile on your line is - 20 Mbps
still getting twice the speed with other testers and net monitor is showing twice the speed.
Gerald

[#7]

Can you carry out the further testing it refers to at the bottom?