stacking combiners produces a strange result

deletetable test
createtable test
setiter -t test -p 16 -scan -n test_1 -class org.apache.accumulo.core.iterators.user.SummingCombiner

count:a

STRING
setiter -t test -p 17 -scan -n test_2 -class org.apache.accumulo.core.iterators.user.SummingCombiner

count:a

STRING
setiter -t test -p 18 -scan -n test_3 -class org.apache.accumulo.core.iterators.user.SummingCombiner

count:a

STRING
setiter -t test -p 10 -scan -n test_4 -class org.apache.accumulo.core.iterators.user.SummingCombiner

count

STRING
insert row count a 1
insert row count a 1
insert row count b 1
insert row count b 1
insert row count b 1
insert row count c 1
scan

row count:a []    2
row count:b []    3
row count:c []    1

row count:a []    12