What is the difference between theoretical and actual food cost?

Theoretical food cost is what your dishes should cost based on costed recipes and what you actually sold — the perfect-world number where every plate uses exactly its recipe. Actual food cost is what the P&L shows you really spent, worked out from opening stock plus purchases minus closing stock. Theoretical is the target; actual is reality. The distance between them is the part of your food spend that left without being sold.

From Restaurant Food Cost Control: Theoretical vs Actual, and How to Close the Gap — the full reading, with the method behind this answer.