For general irreversible deposition processes, a relation between the variance sigma(2) of the number of deposited particles on subsystems out of a large surface and the available surface function Phi is obtained. This relation is based on a mean field assumption and follows the resolution of a master equation system. It is valid at low to intermediate values of the surface coverage theta as shown by comparison with exact results and with numerical simulations for special deposition models. In the low coverage limit, if the available surface function is written as a series expansion of the coverage theta, whose first nontrivial term varies as theta(k), the reduced variance has a similar expansion, However, the prefactor of theta(k) derived in this article is in general different in both series expansions. This result has also been obtained by a rigorous argument based on the evolution of the k-particle distribution function with the coverage.