The metadata search command is not time bound. As per documentation for metadata search command:-
The metadata command returns data about a specified index or distributed search peer. It returns information such as a list of the hosts, sources, or source types accumulated over time and when the first, last, and most recent event was seen for each value of the specified metadata type. ****It does not provide a snapshot of an index over a specific timeframe (such as last 7 days).****
So what | metadata index=myindex type=sources is showing, is for all time (always) and what index=myindex source=source1 | stats count is showing is just for your time period (April 1st, 2014), hence the difference.