Windows 7 defragmenter automatically analyzes SSD

I have newly installed a Samsung 830 128GB SSD as my system C: drive and installed Windows 7 freshly on it.

Windows 7 recognized it as SSD after running WEI and excluded the drive to be defragmented (the C: drive is not shown in the "Disk Defragmenter -> Configure schedule -> Select disks").

However, Windows 7 defragmenter seems to automatically analyzes the SSD frequently in system idle (3 times from 1am to 4am last night). The Disk Defragmenter shows Last Run of the SSD with the "% fragmented". I noticed the value was different, smaller than that of two days ago. Also, in the Event Viewer, it shows the three log of "The disk defragmenter successfully completed analysis on SYSTEM_SSD (C "

Any idea why Windows 7 does that?