KBNKNN also has a negative number in the tbs statistics.
What I've been using these statistics for, is coming up with my own priority for which table-bases to download. In addition to frequency (taken from posts linked on the Endgame Tablebases Online site - I use a 2:1 weighted mix of human and computer frequency), I wanted to take into account file size (if two tablebases occur with the same frequency, but one takes up 11 GB and one takes up 2 GB, having the 2 GB is obviously more efficient, as it leaves 9 GB available for more tablebases), and how much impact the endgame has (e.g. KBNKBN is almost always drawn, and if it's not, it's likely to be due to tactics in the very first moves following conversion, which the search can easily find - contrast against KPPKPP which is usually decisive, so much more important to have knowledge of). Of course, the relevance of these can be disputed, and the method in which I used the available statistics is not necessarily the best one.
Still, whereas I did this for my own personal use, perhaps others can find the results interesting as well. For full details, see the attached spreadsheet - I have not done the statistics for all the tablebases (and don't know if I'll bother with the rest, as the remaining ones have very low occurrence frequencies, so I won't download them). Actually, turns out I couldn't attach the spreadsheet - "The extension ods is not allowed" - but I'd be happy to email it to anybody curious.
So, here is my personal order to download them in, along with the effiency rating my spreadsheet gives for each of them:
Efficiency Ending
100000 kppkpp
22657 kpppkp
18391 krpkrp
10591 kpppkr
8044 krppkr
7960 kbpkpp
6686 knpkpp
3799 kpppkb
2652 kpppkn
2649 krpkpp
2289 knpknp
2216 kbpknp
2105 kbppkb
2058 kbpkbp
1657 knppkb
1635 kqpkqp
1493 kbppkr
1450 krpkbp
1448 knppkp
1184 knppkr
1116 knppkn
1059 kbppkp
953 kqppkq
691 krpknp
662 krppkq
600 kpppkq
479 kbppkn
447 krrkrr
439 krbkrp
299 kqrkqr
296 krrkrb
284 kqqkqq
249 krnkrp
241 krrkrn
220 kbnkpp
219 krbkrn
208 kqpkrp
195 krnpkr
190 krbpkq
180 krbkrb
163 knnpkr
147 krppkp
137 kbbkpp
132 krbpkr
130 kqpkrr
127 krpkbn
126 krrkrp
116 krnkrn
115 kbbpkr
110 kbppkq
109 krrpkq
104 kbnpkr
101 krnpkq