If a germ cell or somatic cell had the original mutation, then there's an increased risk for cancer in other parts of the body too. If only retinal cells in one eye is mutated, only that one eye is at risk for cancer.

This is what is known as a somatic mutation: a mutation that occurs in non-germline cells and, therefore, only effects daughter cells derived from the parent cell with the mutation. So, a mutation occurred in a differentiated parent cell that gave rise to a set of daughter cells (which, in this case, were destined to become retinal tissue).

no family history of neoplasia and unilateral neoplasm suggests sporadic form of retinoblastoma (vs germline (familial) form which is associated with bilateral retinoblastoma and osteosarcoma

in the sporadic form mutation occurs at the site (retinal cells) vs germline (familial) form mutation occurs in germ cells