i think i found the problem.
this does not happen when i copy a file. sorry for misleading you guys
actually it looks like x-plore does not handle timestamps for the files in root folders. be it anything but sdcard - /data, /system, or /cache... every file in root folders has wrong timestamp.
even the /data/app/com.lonelycatgames.Xplore-2.apk (yes, the most recent x-plore app), which i updated about two hours ago, is shown to have been modified on 8th december, 2006.
i installed other file explorer software to compare the issue, but root explorer did properly display timestamps for root files. so i think my root files have proper timestamps actually, but they are not read or displayed properly in x-plore.