I am only thinking aloud, dxb; don't know for sure.

It seems probable that there are two separate Latin roots, because the *pattern of the mesh is different in both. A reticulum is a fine meshwork of assymetric branching lines that connect randomly and without a predetermined design. Whilst with a graticule, the grid design is extremely ordered, regular and symmetric.

None of this explains why the manufacturer of the optical instrument decided to use the terms as synonyms!? Maybe, if you went into the history of the instruments themselve? I didn't know this second usage before; thanks!