Generally speaking, when you run a powershell command it only shows what sections are deemed as important. If you take the same command and pipe it to format-list (or "ft" for short) you will get everything.
Get-Volume | ft
When exporting it exports everything.
Also, you need to add the paramater -NoTypeInformation to get rid of the first row.
To only get certain values, you will just pipe it using select.. something like this:
Get-Volume | select HealthStatus, DriveLetter, SizeRemaining,DriveType | Export-Csv -NoTypeInformation -Path C:\temp\software1.csv
Also, there is no need to do $results = get-volume... This pushes the output into the variable $results. This would be applicable if you wanted to recall the variable later. So, you could also do something like this..
$results = Get-Volume
$results | select HealthStatus, DriveLetter, SizeRemaining, DriveType | Export-Csv -NoTypeInformation -Path C:\temp\software1.csv