It uses a static value, generally determined by EJ at the start of either a major content patch or expansion. That value changes based on gear and usually set to 'end game heroic' gear. Most people don't get to that point. Those values don't become relevant until that gear level is reached.
This actually isn't right. We run exhaustive tests with SimC to generate our first set of weights before a patch comes out. Then we turn to theorycrafters, bloggers, people on EJ, etc, to test those weights out. We work with them to fine-tune the weights and bam - you have our default weights. These weights are meant for the average person - to cover the widest range of people who raid (including LFR). They are not set for end-game gear.
Then as people's gear improves (we we can track with average iLevels), we continue to work with theorycrafters and add additional preset weights. Sometimes we change the default to a new set if people's overall gear is improving that much.
A little more about using SimC / ShC to get custom weights. This is AWESOME, if you know what to do. You see, when you get your weights, re-gem, re-reforge, etc, and re-sim those results, you'll see a NEW set of weights. If you continue to do this you end up in these loops where the simulator is always changing your weights. We account for that, and wrote a program that does this over and over and over until we get a set of stable weights that work for a majority of the people.
Side note: We LOVE simC and ShC, we use them all the time. We encourage people to use them. They are awesome programs and they simulate DPS. Mr. Robot optimizes gear, and doesn't simulate DPS. (But we do have an export option to send to SimC).
Problem with shadowcraft is it has so much potential but doesn't fulfill entirely:
It provides stat weightings which are excellent and convenient, but nothing simc can't handle (plus simc is more flexible).
It provides an estimation of dps, which is usually a fair bit under simc and real world examples (maybe potential vs average)
It provides a reforging tool which is very heavy handed on secondary stats, i.e. swings from full haste to full mastery. Though it is good for hit and expertise caps by finding ways of minimizing excess.
It provides a very poor method of ranking items in one list, if anything this can only be used to see what other options are available.
AskMrrobot suffers from static weightings but makes up with solid ranking lists and a more complete reforging experience (locking specific items, gems you want, etc.).
Both make for good reforging with a little work. It's a shame, as it's super exciting to see love for a class so void of attention but it does need some moderate refinement to remove the annoying design flaws.
I had problems with Mr. Robot, too. When I optimized my gear it told me to reforge almost all of my expertise and still gave me 7,5 per cent. Yet, when I reforged ingame it was almost 1 per cent too less. You should always keep an eye on that yourself.