The London number is for Inner London, while the Paris figure is for the City of Paris, which is the equivalent of Inner London. So they are comparable right? Wrong. In Paris, the bulk of the upper middle class lives in the inner city and the poorest areas are in the suburbs. In London, far more of the upper middle class lives in the suburbs and the poorest areas are in the inner city. Since Paris only marginally beats London on a comparison that is loaded in its favour, I would strongly suspect that the London urban area as a whole is richer than the Paris urban area as a whole.
Frankfurt is really too small for comparisons with London and Paris to be very relevant.