That could be because the data could have been already purged for last half.
Go to settings and click on Indexes:
What is the Max size (MB) of entire index set for _internal index??
You will also see a column - Earliest event ..what is the earliest event there?
yes the _internal index retention are 30days and 500Mb, so if may have deleted the oldest events already if the size reached 500 in less than one month.
The max size is 500mb, the current size is 905mb. The earliest event is 6/22/15, and the latest event is 8/21/15 current time. Could it be the size?