(Mini)USB keyboard has D- default high
A lot of keyboards support PS/2 for backwards compatibility - they detect whether connected to a USB host, and if not will behave as a PS/2 keyboard.

It is possible this is what you are seeing - if you haven't connected anything to the D+/D- lines then the keyboard would determine it is not connected to a USB host and fall back to PS/2. If it didn't have this mode, you would probably see nothing at all on the data lines as there is no host connected to issue commands.

There are USB to PS/2 adapters which are basically just passive and connect the D+ and D- signals to the clock and data signals of the PS/2 connector. Might be worth trying one of these (if you have one lying around and a computer which has as PS/2 port) to verify.