Basic Speeds

[#1]

I'm trying to learn a little more about the background technicalities of BB. I understand an increased amount of people using the same exchange at the same time will reduce the speed etc and the distance from the exchange plays a part as well the state of the copper . What I don't understand is why the speed can and does vary over a matter of weeks.
A couple of years ago my speed on average was 750kbps I had a steady decrease and after 2 months BT changed my master socket - my speed shot up to 3Mb despite BT saying I could only expect 1Mb. However soon after that BT upgraded the expected exchange speed to 3.5Mb.
Since then on average I've achieved 3.5Mb which basically I am happy with. Sometimes it has risen to 4Mb but 2 weeks before Christmas and up to last week I have been getting 4.5Mb, last week tho' it has dropped back to 3.3Mb. So why would it sustain an increase for 3-4 weeks then fall back?
I don't see the point in raising a ticket as I'm sure I will told your speed is within limits but a decrease of 20% or so at those speeds makes a lot of difference.

[#2]

We do advise that broadband speeds can vary slightly and this can happen for a number of different reasons. Environmental factors and the weather, especially extreme changes in temperature and heavy rain and faults on the telephone network between you and the exchange can cause this.
With regards to your personal experience I'd be a little concerned by a change from 4mb to 3.3mb and I'd consider that an issue if it happens again rather than a technicality of broadband in general.

[#3]

Thanks for help Adam, I'll keep an eye on it .