A filesystem being mounted as either ext4 and ext3?

I came across a very interesting problem on my computer today when moving my Dropbox folder as Dropbox is dropping support for non-ext4 filesystems on Linux now. I wasted a lot of time on this as I always believed that this partition was in ext4. So I couldn't figure out why I couldn't get expected by following instructions.

I could mount a partition on my hard drive as either ext4 or ext3 format. Dropbox's daemon is not stupid. If the partition is mounted as ext3, it will complain.

So here is the details. If I do not specify the format, it will be mounted as an ext3 partition.

forrest@test:~$ sudo mount  /dev/sdd1 /mnt/test/
forrest@test:~$ mount | grep sdd1
/dev/sdd1 on /mnt/test type ext3 (rw,relatime,data=ordered)

If I specify the ext4 format, it will be mounted as an ext4 partition.

forrest@test:~$ sudo umount /dev/sdc1
forrest@test:~$ sudo mount -t ext4 /dev/sdd1 /mnt/test/
forrest@test:~$ mount | grep sdc1
/dev/sdd1 on /mnt/test type ext4 (rw,relatime,data=ordered)

I don't know why. But it confused me quite a bit.