Maybe I'm blind to evidence but why should they be different?
Phones are single-user, tablets can support multiple. Tablets are more often out when unused, phones are typically in your pocket. Phones are always with you, tablets are more likely to be left at home.
This looks like putting the cart before the horse to me. One can say "we have one-user UI and multi-user UI", etc. - that's good. But saying "this is a tablet so you always get multi user UI and this is a phone so you always get single user UI" looks wrong for me - the purpose of the user should be driving the UI, not the other way around. Of course, maybe the whole point of Apple is to drive user's choice and thus make them comfortable by not confusing them with too much variety and options. I see how this can appeal to some people though I am definitely not one of them. That's why the only Apple device I've ever owned - an iPad - has been collecting dust in a desk drawer since the minute I've got an Android tablet. I guess these are just two different models of user interaction that appeal to different kinds of people.