。
Theory doesn't necessarily match practice.
Literal
Theory [in-では] practice [with-とは] necessarily match not.
理論では (in theory / within the realm of theory) sets up a contrast with reality. 必ずしも~ない is the partial negation 'not necessarily / not always.' 一致する (to match / agree) is the formal verb for abstract alignment between two things. A measured academic claim with built-in hedging.