The thing is that the Python from Python.org is 32 bits.
The python installed with mac os 10.6 is 64 bits.
I checked that using :
import struct struct.calcsize("P")*8
Just for the record, the python installed with Mac OS is not just 64-bit, it's (like all other Snow Leopard binaries) a 3-way universal binary containing the executable for i386, x86_64, and ppc (32-bit) architectures (on Leopard, it was full 4-way universal binary - i386, x86_64, ppc, ppc64). It's just the policy of the OS to choose x86_64 variant of an executable when more than one architecture is available in the universal binary, and the default policy can be changed for a specific command using the "arch" command. The 32-bit variant returns "32" (launched with the command: arch -i386 python -c 'import struct; print struct.calcsize("P")*8'