“Much like how Champagne can only be called champagne when grown in the Champagne region in France, San Marzano tomatoes must be grown in the Agro Sarnese-Nocerino region of Italy to be called such. When the tomatoes are canned, they are required to have a DOP emblem on the label to symbolize their authenticity.” [wideopeneats] Here is a link to the full article and it is an eye opener. An excellent read. San Marzano Tomatoes Fake.
From the Cento website, (Cento Tomato Products)

Cento is the only United States brand that owns its production facility in the Sarnese Nocerino area of Italy, literally in the shadow of Mount Vesuvius. The rich volcanic soil, high water table and ideal Mediterranean microclimate all combine to make San Marzano tomatoes the premier choice for any recipe.

And why are the Cento San Marzano tomatoes so popular in many of the larger professional kitchens?

San Marzano tomatoes are widely recognized by top chefs, foodies, Italian cooks and food aficionados as the gold standard for taste, but what makes them so special? San Marzano tomatoes get their name from the town where they were born, San Marzano sul Sarno, which is located in the Campania region of southern Italy. Characteristics of San Marzano tomatoes include a thicker tomato wall, less seeds and less acidity than other tomatoes, making them ideal for authentic Italian cuisine. San Marzano tomatoes thrive in the designated area of Italy because of the Mediterranean microclimate, high water table, and fertile volcanic soil. Truly authentic San Marzano tomatoes from Italy adhere to strict conditions and guidelines in terms of their growing, selection, and processing. Certified San Marzano tomatoes must be obtained from plants of the same ecotype, grown within a specific territory allowed in Italy, and contain characteristics that comply with standards set in Italy. [Cento]

Watch some of the Food Network Italian and Tuscan chefs, and you will see that they use Cento products. There is a reason and it is stated above. We always have some on our shelves.