You sit too high in minecarts. About a block too high.
You should be sat so that your chest is in line with the carts' top.
Instead you are floating way above that with you sitting in mid-air.
Steps to Reproduce:
1. Place a minecart
2. Get in it.
Environment
Put your operating system (Windows 7, Windows XP, OSX) and Java version if you know it here
Duplicate of MC-47448