Ubuntu reformatting can’t be read in Windows 7

When I reformat a USB stick on my Ubuntu 11.10 system, my Windows 7 computer can't read the flash drive.
I tried it with Disk Utility, gParted, & mkfs (mkfs.vfat / mkfs.ntfs) but nothing worked.
However, when I format it via my Windows 7, I can read and copy files through Ubuntu and then see them with Windows 7.
Why is this so?
Please explain it to me.
Thanks!
