couch_docs_fragmentation is reported over 100%

First reported here: https://issues.couchbase.com/browse/MB-55268?focusedCommentId=660111&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-660111

We're seeing very high (over 100%) value for couch_docs_fragmentation in some of the perf tests.

I found a run that has cbcollects: http://perf.jenkins.couchbase.com/job/rhea-5node2/3193/consoleFull

The access phase shows couch_docs_fragmentation as 140%:

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=rhea_720-5212_access_f3d6#798db49106ccb540caf8b64b5c4e31f2

I checked the cbcollect for all 4 nodes looking at stat (db_data_size, db_file_size) - plotted as the first chart (left most). They look ok to me. DB size is around 1285GB Data size is 860GB. So the couch_docs_fragmentation should be 33%. 

Since the computation happens on ns-server side, request the team to take a look as the source stats from kv/magma side look fine.