This is pretty much standard. The numbers of the product - dlan 200 and dlan 500 - are the nominal value (i.e. "up to..."), you will never achieve those speeds over an extended run of mains cabling.
Having said that, the speeds you are getting are a little low when going from upstairs front to downstairs back, although you should still be able to stream HD video. For comparison, I am getting 384 from my upstairs office to downstairs lounge, and 170 out to my external garage. If you are using extension leads, this can impact on your speeds, you want to plug the adapters into the mains socket if at all possible.