There is a public JRE component inside JDK installer, I did the following test on one machine:
1. Install JRE 6u5 on that machine.
2. Install JDK 6u10 on the same machine, the public JRE component didn't show up during installation, it is thinking the machine already has the latest JRE 6u10, which isn't correct.
I tested on both 32bit and 64bit OS, with 32bit JDK and 64bit JDK, the same issue has been found. It is important to fix this issue in 64bit OS because it will stop to install 64bit JRE when you have a 32bit 6u5 JRE installed on user's machine.