What should a PKG do if replacing existing files?

Heres an oddity…
I recently started wrapping my app into a pkg

I installed the 64bit build and tested.
To switch between 64 bit and 32 bit builds, I deleted the 64 bit app by putting it in the Trash
Then ran the pkg for the 32 bit version.
Nothing showed up in Applications.
Emptied trash, re-installed, nothing in Applications

I had to manually create a file of the right name in Applications before the PKG would unpack the new version there.
Any idea whats going on?
I figured the pkg would just ‘save into Applications’ regardless?