Because only the iPhone 4 can be compared with our SGS, IMHO. They came out the same year and have the same CPU clock and RAM.
iPhone 3GS has lower specs.
Think about it this way : the iphone 4 was released in 2010 and recently released an update in 2013, so it received more than 3 years of updates.
Our Vibrant was released on July 2010, and has received the 2.2.1 update, which was out January 2011. So, half a year of updates.
Nexus 4 16GB
Old phone: Samsung Vibrant 16GB