Transportable aplies to the CPU, not the operating system.

You don't need to use transportable between Operating Systems that use the
same processor or a compatible one (Just like Intel and AMD), but you will
need to use it if the processores are different (Alpha, for example).

The "problem" is related to the way integers are stored/used: some
processors reverse the bits.

It's really "save" to move a database form one operating system to another,
with no -t parameter, as long as they use the same processor arquitecture.

Artur