Report Writer timeframe of Last Month... is it really last month or more like last 30 days???

The question i have is that I always assumed reports run from report writer with timeframe of last month would be on the data from the previous month... the problem I've run into is that on one of my peak and average network utilization reports... when i run it a couple times during the following month... the data isn't the same.  I would think that previous months data would be static ie. not change.  Am I wrong about this?  Is it really more like last 30 days than last month???

Here's an example:

Yesterday:

Today:

I would think that if the data was really "last month" that running the report two days the next month should show the SAME data.  Does anyone have any insight into this????