Because on the front page it says 32 bits is for less than 2Go...
If your CPU is 32 bit then you would install 32 bit Ubuntu.
A 32-bit OS will run on a 32-bit or 64-bit processor without any problem.
A 32-bit application will run on a 32-bit or 64-bit OS without any problem.
But a 64-bit application will only run on a 64-bit OS and a 64-bit OS will only run on a 64-bit processor.