I believe from Porta Garibaldi to MXP is also a bit faster than from Milano Centrale. But really, these differences are not such a huge deal. Depends where your train arriving in Milan stops, or does not. Typically, some stop at one station, some at more than one.

Some years back, on my way to a meeting near Pisa, I had bought a non-refundable train ticket from Milano Centrale. There was a huge line at passport control. No time to buy a ticket to the airport train. I barely made it to the last train that would let me connect to my booked train at Milano Centrale, thinking I would need to buy on board or whatever. In the end the conductor never showed up and I never paid.