Lemon zest and lemon juice do different work
The idea
Lemon juice supplies liquid and sharpness; the colored outer peel carries a different citrus aroma. Adding one does not reproduce everything the other contributes. A dish can smell strongly of lemon without having an equally sharp taste, depending on which parts of the fruit were used.
A practical next step
When following a recipe, keep zest and juice as separate ingredients. Avoid treating a splash of juice as an exact replacement for the peel.