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OK, I think I should indeed have explained this in the readme file. Note there are two kinds of output: what is printed during the building, and shown in the WinBoard Engine-Output window (if you are lucky), and what is written on the file rep2.txt. (Silly name; I should also change that...) The former is raw statistics after symmetry reduction. The rep2.txt statistics is the true statistics, where every position that had a diagonal mirror image different from itself which was not taken into account in the calculation to save time is counted twice. But the numbers printed are still only counting positions where the King is in the square a1-d1-d4-a4. But this always would give you a factor 4, as on 8x8 boards there never are any horizontally or vertically symmetric positions (if you have one King, that is).

Now the meaning of the output:
TOTAL = the number of positions with no two pieces on the same square
WON.wtm = nr of positions won to white with white to move
K captures = nr of the above where white can capture the black King
other = WON.wtm - K capture

Then follows a table of the number of positions that are won to white when black has the move, split out by how many moves it takes to effect the win. In (default) DTC mode this is offset by 10, so 10 means black is currently checkmated, 11 he will be mated in 1, etc. (Losing a non-royal defender is counted in the same way as losing the King, so the count already reaches 10 when the piece will unavoidably be exposed to a winning capture on the next move, and not when you have actually captured it. The position after the capture would be listed as 9, but is not listed at all, because it belongs to a different tablebase, with another number of pieces. Nevertheless, now that I think about it, this is a bit weird, and it might be better if I change it such that I count positions where the non-royal piece is 'mated' as 11, or even 12, so that it would no longer prefer to be checkmated over losing its piece and be checkmated in a subsequent phase.) 

The 0-entry of the table gives the number of positions that are not won to white (still with btm) for other reasons than black capturing the white King (e.g. black capturing a non-royal white piece, or having a fortress draw). It probably should be renamed 'nonwins'

WON.btm = the sum of all entries other than the 0-entry in the table
stalemate = number of positions where black to move is stalemated
LEGAL = WON.btm + nonwins + stalemate
W check = positions where the white King is in check (i.e. illegal with btm)

Interpreting the numbers is a bit tricky, as even dead draws usually have most positions won for white to move; a generally won end-game really have a number more than 99% of the total there. To interpret the number, the K-captures help. E.g. in the sample output for KBN_K shown below, you see that more than 25% of all positions is won to white through immediate K capture, which has absolutely nothing to do with the mating potential of the white pieces. If black would have had a piece beside King, (e.g. KBN_KB) there are approximately as many positions where that piece is hanging. (This is not separately listed, but the chances it is protected by its King are only about 10%, so the K-capture number is a good indication for it.) So you would already have about 50% 'false positive' WON.wtm positions that don't have anything to do with the end-game under study.

It is often more revealing to look at the black-to-move numbers. But there you have a large number of false negatives, where black starts with capturing a hanging attacker. The 'W check', where he captures the white King has already been taken out of the WON.btm, (and this is about 10% for an orthodox King on 8x8) but the probability black attacks each of the other white pieces is the same, and they are unprotected more often than not (and sometimes protection does not help, if attacked by a non-royal defender). So it is quite normal that 30-60% of the btm positions are not won in a generally won end-game.

What also helps is to look at the distribution of the WON.btm positions. Even KR_KQ has won positions (for the Rook), but none of those takes more than 3 moves. So the very fast wins should all be discounted as tactically non-quiet positions, that after the tactics raged out don't really belong to the end-game under study. So what really counts is the number of lengthy wins with btm.

KBN_K

WON.wtm    3798926
K capture  1093380
other      2705546
  0.        636982
 10.           116
 11.            78
...
 43.           740
WON.btm    2797042
stalemate     3222
W check     375010
LEGAL      3437246
TOTAL      3812256