The maximum ram supported by Apple is 16 GB. Crucial and OWC have teested and guarantee it will work with 32 GB ram. Crucial and OWC standby their ram...I would not attempt to go to 32 GB with ram from anyone else, however.
What that is saying is that at the time Apple released your Mac for sale, they had successfully tested it using up to 16 GB of RAM (4 x 4 GB modules). Crucial is saying, correctly, that since then, larger 8 GB RAM modules have come on the market and they have successfuly tested your unit to use the full 32 GB (4 x 8 GB modules).