You start with the ions and combine them to create the lattice. This process produces energy. If ions were just point charges, that would be the only process present, and the energy produced would be exactly the lattice energy. However, when ions combine they create a compound, which is in part covalent, Thus the energy produced can be split into two parts (or two stages of the ion combination) - one is responsible for the creation of the covalent bond, the other is responsible for the creation of lattice from the product of the first stage. We are not able to measure these things separately, so the lattice energy calculated from the BH cycle is overestimated by the first part.