Possibly because by having a margin on expenses as well, it keeps the overhead day rate down, making the pricing more competitive? I've certainly seen that kind of contract pricing, where in a competitive environment, headline (staff costs) are kept as low as possible, but expenses may be slightly increased..

It's all murky in consultancy

And finally, in contract negotiations, it's difficult to argue about day rates (they tend to be) or length of time required, but much easier to negotiate on expenses...