The “current” report checks the current status of opportunities from opportunity table where as the “progressive” considered the all the various stages from the opp history table. This includes opportunities that are “Closed (Won or Lost)”, since they demonstrate accomplished work.
The “current” report will tell you about the opportunities currently in the opps table which were created or updated during the specified date range period and that have a specific Stage/Activity currently.
The “progressive” report will tell the count of all the opportunities which had that specific Stage/Activity during the selected period. So if an opportunity changes activity 3 times during the period, it will be counted once for each of those 3 activities. (This tells how may quotes you submitted for example during the month.)
When you do not specify the date range, it defaults to the current month (based on updated date not on follow up date).