That's correct but the demographic differences between the two are highly relevant in sponsorship terms. One of the factors that has to be taken into account in deciding whether a transaction is at fair market value is "the geographic spread of fan base, including elements which may be a particular appeal to particular partners (such as demographic information)."
This requires an assessment of not just the club's fanbase but the sponsor's area of operations. If City got a huge sponsorship deal for instance from a milk marketing company, it would be not especially relevant to rely on how big the club are in Japan where 70% of the population is lactose intolerant. It would be less relevant to rely on City's exposure in South America if the Abu Dhabi Bank doesn't do any business there. On the other hand, exposure in the USA might be regarded as significantly more valuable in sponsorship terms because the average disposable income of supporters in the USA is significantly higher than in, say, south or south-east asia.