Netflix uses several different adaptive bitrate streaming systems. The idea in all these systems is to have a given movie available in many different quality levels, and switch adaptively between them according to network conditions, so as to maximize the quality of service. In other words, to give you the best possible picture quality while still managing to stream properly (i.e. no buffering).
The Silverlight version that runs in your PC's web browser is powered by Microsoft IIS Smooth Streaming.
The technology that powers other playback devices like Roku, Archos, or other embedded devices like your BluRay player is certainly different.
There are a lot of other adaptive streaming systems out there, including Apple's "HTTP Live Streaming" aka HLS, and some offerings from Adobe as well. There's also the legacy "non adaptive" way of doing things, like YouTube does.
Suffice it to say, Netflix actually encodes each movie MANY MANY times, since they have to cover a wide range of bitrates, video compression formats, and encapsulation / adaptive streaming methods.
At the end of the day, you're seeing different streams on your PC then you're seeing on your BluRay player. It's being delivered to you by entirely different code running in a different platform, and being presented in a different way.
Most of the time it all works nicely, but from what I understand, their Silverlight product is the oldest and least advanced of all their devices. The PS3 is supposed to be the most advanced.
The issue you're describing (stuck on a low bitrate) shouldn't technically happen, since all their methods will adaptively switch up and down according to network conditions. If it does happen, then either they are unable to measure your available bandwidth properly, or there's other network traffic that would prevent delivery of a higher bitrate stream.
