Performance test went wrong and the dd command created 13TB of data on /dev/mapper/device. Why system didn't crash? HDD-250GB

So I wanted to do some performance test with encrypted and normal data storage on my embedded device.

That is not what I was expected to see at all!

Can you please explain it to me what just happend. Why dd comand output was 1843200+0 records in and out but df -h show file system disk space usage as 13TB?

This is my workflow:

dd if=/dev/urandom of=enc_per_test.img bs=512 count=2097152

dd if=/dev/urandom of=normal_per_test.img bs=512 count=2097152

And receive 2 images 1GB each - as I predicted.

losetup /dev/loop1 enc_per_test.img 

losetup /dev/loop2 normal_per_test.img

After that I perform:

dmsetup -v create enc_per_test --table "0 $(blockdev --getsz /dev/loop1) crypt <crypt_setup> 0 /dev/loop1 0 1 sector_size:512"

mkfs.ext4 /dev/mapper/enc_per_test

mkdir /mnt/enc_per_test

mount -t ext4 /dev/mapper/enc_per_test /mnt/enc_per_test/

As I expected df-h showed mounted enc_per_test:

 Filesystem ############## Size ### Used ## Avail ## Use% ### Mounted on #####           

 /dev/mapper/enc_per_test ## 976M ## 2.6M ## 907M ## 1% #### /mnt/enc_per_test

I clear cache:

echo 3 > /proc/sys/vm/drop_caches

And finally perform dd comand to fill up the enc_per_test:

time dd if=/tmp/random of=/dev/mapper/enc_per_test conv=fsync

1843200+0 records in
1843200+0 records out
943718400 bytes (944 MB, 900 MiB) copied, 152.098 s, 6.2 MB/s

So I was like, ok that's fine. This is what I wanted. Let's see how it's look like in df -h:

 Filesystem ############## Size ### Used ## Avail ## Use% ### Mounted on #####           

 /dev/mapper/enc_per_test ## 13T ## 13T ## 0 ## 100% #### /mnt/enc_per_test

What happend here?

Thank you!