Debian votes using the Schulze Method, which is a Condorcet method..

Condorcet methods attempt to answer the question "who would win if we asked everyone to vote on every pair of candidates". Most of the variations between methods deal vary only where there are cyclic preferences e.g. people vote B over A, A over C and C over B.

They're broadly considered the fairest voting systems available and they permit a candidate that everyone thinks is okay to win over polarising candidates (unlike first past the post and IRV, but like approval voting).

Back to the numbers, the big table is the reconstruction of the pairwise votes (people actually vote by ranking candidates but the ordering is taken to show your pairwise preferences)

Below that is the Schulze process steps being carried out. At the bottom the "Schwartz set" is the set of top, cyclic candidates. There is no cycle so there is only one candidate remaining and therefore a Condorcet candidate wins (the one people in general vote over everyone else).