Can someone explain the unintuitive ordering in the metallic activity series?
The metals in the activity series are ordered by their oxidation potentials; there is nothing random about it. The displacement reactions, though, are but one of the applications of these potentials, and not the most important one at that. Moreover, with active metals it does not make much sense anyway. The oxidation potentials are defined for water solutions; their significance ultimately stems from the fact that we live in a world dominated by water. Well, you can't react lithium or rubidium with anything in water, because they would rather react with water itself. And if you try a reaction in other media than water, the oxidation potentials do not apply and the outcome can't be predicted that easily. For example, magnesium would reduce sodium from its oxide, despite being nominally "less active".

Indeed, the very concept that some things are "more active" than others is just an approximation; admittedly, a pretty good one when we talk about metals, but still an approximation.