num_docs_indexed stat does not get updated correctly

num_docs_indexed stat does not get updated correctly. Once Index build is started,
num_docs_pending stat is updated regularly but num_docs_indexed stays 0 for a long time. In the stats snippet below, it can be seen that the stats show that 1.6M items(total 2M item test) are indexed instantaneously.

"index/num_docs_indexed":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1695045.045045045,0,0,0,0,0,154439.4394394394,0,0,0,0]

Logs were uploaded to the following URLs:
https://s3.amazonaws.com/cb-customers/couchbase-performance/14626/collectinfo-2015-04-20T223223-ns_1%40172.23.100.16.zip
https://s3.amazonaws.com/cb-customers/couchbase-performance/14626/collectinfo-2015-04-20T223223-ns_1%40172.23.120.18.zip
https://s3.amazonaws.com/cb-customers/couchbase-performance/14626/collectinfo-2015-04-20T223223-ns_1%40172.23.120.19.zip
https://s3.amazonaws.com/cb-customers/couchbase-performance/14626/collectinfo-2015-04-20T223223-ns_1%40172.23.120.20.zip
https://s3.amazonaws.com/cb-customers/couchbase-performance/14626/collectinfo-2015-04-20T223223-ns_1%40172.23.120.21.zip