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"Credits" are rewarded to users to signify their computation contributions. BOINC has given a specification for how much one credit is supposed to be worth, but it's up to each project to implement their calculation of credits. Unfortunately (long story short) some projects have failed massively to follow the spec and/or provide a credit computation that can be reasonably comparable to other projects. Additionally, CPU work units and GPU work units are generally not comparable. This is mostly because credits are calculated based on FLOPS. Since GPUs perform highly parallel computations, they always have more throughput and therefore yield more credits than CPUs. CPU computations are still a valuable contribution, as many projects cannot be (or have not been) effectively parallelized, but unfortunately this is not reflected in credits.
Bottom lineAs a result, the BOINCstats leaderboards are effectively broken with regards to directly comparing credits. For those interested in looking more closely at the leaderboards, Bitcoin Utopia and Collatz Conjecture are examples of projects that do not give reasonably comparable scores. As a consequence the leaderboard tops largely consist of people whose credits are mostly made up of these two projects.
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