Could be that while you're pulling down at 256k, the proxy actually gets the file faster than it can give you, then when it received the entire file, it starts giving it to you at full speed.
I think if they changed it to work like that, that they have a pretty good formula for a greatish product.
However, keep in mind Sentech has now released their service in Nelspruit, so what you might be experiencing is extra bandwidth Sentech purchased, because the way the bandwidth manager works, it tries and distributes all the bandwidth available.(unless they changed it since august)