Actually they are technically the same. When Mockito was first created, we were talking about stubs, so the vocabulary followed that idea. Later people thought it was better to think in interactions rather that technical terms, so the vocabulary followed the when … then … style.
This change in vocabulary helps people to think about interactions, messaging between object. Which is the most interesting idea (message passing) thing in an object oriented language (quoting Alan Kay).
Nowadays testing approach has evolved to Behavior Driven Development (from Dan North), which is almost the same thing but focus even more on the behavior at design time. To reflect that thinking, people asked Mockito to offer an API that reflect that change. So you also use given … will … style from BDDMockito
given(the_type.performs_that()).willReturn(something)
This is my preferred vocabulary now as I use tests to drive my objects design.