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You might refer to one of these exceptions:
Since the first one would happen in a "tour group" (I have no information on whether that is just a front or enforced in practice), I assume you observed the latter one.
Visitors from most countries may obtain an entry visa when travelling to and staying solely in the three Special Economic Zones: Shenzhen, Zhuhai and Xiamen.[59] Visitors can only travel within these cities and cannot proceed further into other parts of Mainland China. Visa for Shenzhen is valid for 5 days, and visa for Xiamen and Zhuhai is valid for 3 days. The visa can only be obtained only upon arrival at Luohu Port, Huanggang Port Control Point, Fuyong Ferry Terminal or Shekou Passenger Terminal for Shenzhen;[60] Gongbei Port of Entry, Hengqin Port or Jiuzhou Port for Zhuhai;[61] and Xiamen Gaoqi International Airport for Xiamen.[62] Visa fees are charged on a reciprocal basis ranging from „168 to „420.
Adding to this info from wiki, you can find more details on the process and cost at TravelChinaGuide (no affiliation, too much text to quote as fair use). A synthesis: