dd didn't stop at partition boundary?

[Man-wai Chang]

I was trying to copy a partiton of 9.7G (/dev/sda1) when mounted with 
partition id being 27 (Unknown) to a file in an NTFS partition (/dev/sda3):

dd if=/dev/sda1 of=/dev/sda3/samsung.bin

dd didn't stop after samsung.bin reached the size of 9.7G and went 
through 13G. I aborted it.

Why?

-- 
   @~@    http://changmw.homeip.net:8080
  / v \   May the Force and Farce be with you! Linux 2.6.31.6
/( _ )\  (x86_64 Ubuntu 9.10)  20:12:01 up 3 days 8:53
   ^ ^    1 user load average: 0.00 0.01 0.00