Also people buying Hong Kong sims for access to uncensored internet by roaming in Mainland China.

In many countries people carry more than one phone/SIM card. Mostly for separation of concerns, i.e business and pleasure.

Also for tariffs across providers. It isn't unusual to have one carrier better for data and another for voice/sms.

Another significant factor is contacts. If your contacts are on one network and you are on another, there is enough motivation to get a second device as intra-network is cheaper than cross-network

Also, an extra SIM card is a useful option in case one network goes down.

Wouldn't a mobile number, a home number and Skype number count?

I wondered if perhaps some of these are also business numbers? Unless they were excluded?