[e1_ZX-9r]

Hard to tell on the angle, but it looks like the flow is right to left. The mid-pipe's outlet diameter is matched to the slip-on's inlet diameter, whatever diameter that turns out to be. It sounds like you figured it out on your own. It definitely looks reversed and that, by definition, would be incorrect. An exhaust pipe's outlet diameter must either be static from beginning to outlet or increase as it makes its way to the outlet. Diameter restrictions can bottleneck exhaust gas flow.

A pic from the exhaust muffler how-to I did a couple years back

 

[e1_ZX-9r]

I'd only be repeating.