That's the way subreports work. You can't get ALL of the values from it, just the last one. To get this to work you would have to link to the subreport the details data in the main report. I would then condense the report down to just one section - details or group, it doesn't matter - the width of the data you want to display in your desired result (so it looks like a field instead of a subreport!) and put the subreport in the column where you want the data to display on your main report. No variables are required.
However, there may be a way to do this without using any subreports - subreports WILL slow down report processing. What do your tables look like? Can you describe what you're trying to do?
-Dell