As a game unfolds through play, metaplay, and transformative play,unexpected things happen, patterns that are impossible to completely pre-dict.
In most first person shooter games, the AI is very predictable and if you pay attention, you can use a specific set of moves or actions to defeat them. However, against a human opponent/groups, players adapt to techniques or strategies. As a result, their opponents either emulate or innovate to defeat them. So the play transforms and changes.