Adsl guides are the best guide and are based on what is happening in real time. From minute to minute, there can be changes in contention retes etc. so differing speed will show up from one time to another. Also to take in consideration is the distance you are from the exchange, which can also cause differing speeds again from minute to minute. Keep using ADSL Guides I do and feel happier that way. Also comparing one speed table with another is no guide as they all measure in differing ways and are not all like for like.
Would you compare a Mini with a Jaguar this time and a Ford Cortina another, to get the best result you should compare yours with the same, each and every time