Ok, i finally figured out what this number means, after working with SSD on Windows 7.
This number means size of data space between the start of physical disk and start of logical disk on SSD that you selected in AS SSD.
So if you install windows 7 on empty un-partitioned SSD it will create first 500 MB service logical disc for win 7 needs, and then System C. Thus why you will have number counting in KB this 500MB logical disk before System C.
And if you created, say, 60GB System C, and selected for test logical Disk D in AS SSD - it will show number in KB of "500MB logical disk PLUS System C".