The user is translating the target's World Space position to the corresponding Cell Position of the Hexagonal Grid. This will correctly give the Cell Position on the Hexagonal Grid.
The Cell Position on the Hexagonal Grid is not always equal to its corresponding World Space position in value. The purpose of the Grid is to help translate Cell Positions on the Grid to its corresponding Local and World Space positions and vice versa.
User should instead be translating the target's World Space position to the Grid's Cell Position and then translating the Grid's Cell Position to the Grid's World Space Position for this case, to get the correct rounded position of the Hexagonal cell.