tl;dr: all of the contributions are based on statistical voting. what that means if you pin a particular percentage of your attention at X% for site1, or if you happen to spend Y% of your attention for site2, then your contributions will approximate those percentages over time.
there are two reasons for this:
first, in order to provide anonymous contributions, the contributions are divided into votes. by your browser. the reason that this is done by the browser is to avoid sending a synopsis of your browsing history over to our server (or any third-party). the downside of this is that if you have 100 votes and you spend 2.1 percent of your time somewhere, you can’t send 2.1 votes. so, when it is time to make a contribution, all of the sites that you may contribute too are given a relative weight from 0.0 to 1.0. so, a site that’s pinned at 40% gets a weight of 0.4 (similarly a site that got 2.1% of your attention gets a weight of 0.021). then a pseudo-random number is generated from 0.0 to 1.0 and a site is chosen using that number. so if the first site in the list had a weight of 0.4, and the second site had a weight of 0.021, then a pseudo-random number from 0.0 up to (but not including) 0.4 would indicate a vote for the first site. similarly a pseudo-random number from 0.4 up to (but not including) 0.421 would indicate a vote for the second site, and so on.
the second reason is that a site that got a very small percentage of your attention, still deserves a chance to get a vote. statistical voting makes that happen.
certainly this isn’t intuitive from the user-interface, and suggestions on how to add wording to make explain it better would be appreciated.
ps: you might ask: well, if i pin a site at X%, why doesn’t X% of my contribution go to that site? that would require that the total number of votes you get be evenly divided by X. say you have 50 votes, and you pin a site at 35%, then do we round-up to 18 votes or down to 17 votes? and what happens when X is really small or really big?