BT's test results says:
IP Profile for your line is - 70.07 Mbps
This is one of the main indicators that we want to get out of a BT speedtest - the IP Profile value.
It tells you that your sync speed is now roughly 72Mbps.
When you were getting speedtest.net results of around 75Mbps, you *probably* had an IP Profile of around 77Mbps, and a sync speed of around 80Mbps.
Why has your sync speed dropped? That is hard to tell, but people can offer advice if you post your router statistics.
However, you need to be aware that FTTC isn't like ADSL - your speed can drop over time, with a primary cause being crosstalk from other FTTC subscribers. As FTTC takeup increases, those extra subscribers interfere with your line, and your speed drops.
It is the primary reason that you are given a speed range. Your range appears to be 66-80Mbps; the lower end of the range is designed to take account of most of this crosstalk. Thus you can expect that your speed *could* drop that far - without BT taking any action to fix things. If your sync speed could drop to 66Mbps, you could probably expect speedtest.net results to drop to just over 60Mbps.
Other reasons are possible - including line faults that BT will attempt to fix. But be prepared for it to really be just crosstalk, and set expectations appropriately.