How to properly factory-reset a Samsung Galaxy S2?
A factory reset usually doesn't touch "external storage" (may sound confusing, but that includes the "internal SDcard" as well) – unless it explicitly specifies so (some devices/ROMs have a corresponding checkbox in the system menu, usually Settings → Backup &amp; Reset, where this might be specified to be included).

What a factory reset usually does is, basically, to wipe /cache (where the application cache should be) and /data (where the userdata reside, including the apps the user installed).

Which means: Nothing is wrong here, it's "working as designed". If you want the SDcard to be wiped, you will have to do so yourself. You can e.g. use a file manager app, or connect your device to a computer. Or simply use a terminal app (or adb shell), and invoke rm -rf /storage/sdcard0/* to have all files removed from there.

See also: factory-reset tag-wiki