Two huge differences through...
IPhone is more popular than the iPad.
IPhone starts at $199 compared to $499
Good luck getting an iPhone two days after its released.
you are making the assumption that Apple produced exactly the same number of iPhones as they did with the previous models, and unless you checked their inventory reports, you can't say that.
the iPad 2 was sold out for MONTHS after launch, iPad 3, which sold even more, was not. Maybe Apple learned something... maybe?