You don't say whether you have a MAX bb product. One of the features of MAX is that the rate varies the whole time. It is most likely to be fastest at the least busy times.
Having said that, your variability is rather wide.
One of the key measures is the downstream rate (synch speed), which you will normally find in your router's statistics. Your download speed is never going to be much more than 80% of this, but could be a lot lower.
Different adsl modems will synch at different rates. My D-Link modem synches 50% faster than my BT.
Its also noted that the various websites that "measure" your download speed can give different answers.
The downstream signal to noise ratio will also affect the download speed. This can also vary over time.
Finally, if you have recently changed over to a MAX product, you should be aware that this is much more sensitive to your house's internal wiring characteristics and also to marginal microfilters.
Try plugging your modem into the telephone master socket and comparing the synch speed there to the speed you get at your normal location. Any difference will be due to your internal wiring.
I have also found that leaving the modem powered up all the time improved my download speeds. BT monitor the synch speed over a 3 day period and adjust the download speed accordingly. It would appear that sometimes switching off the modem confuses their system.