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You've misunderstood the announcement. All the blessings on the megilla reading are for the megilla reading. Each reading is a mitzva and gets blessings. Some communities view the daytime reading ...
Answer
#1: Initial revision
You've misunderstood the announcement. All the blessings on the megilla reading are for the megilla reading. Each reading is a mitzva and gets blessings. Some communities view the daytime reading as a higher level obligation and say shehechiyanu on it too. There are no blessings on the other purim commandments. Why not you ask? Shouldn't we at least say shehechiyanu on those commands? Some later authorities (led by Magen Avraham 692 quoting Shel"ah) suggest saying shehechiyanu on something else (like a new fruit or, commonly and conveniently, a megillah reading) and having those commandments in mind to remove any doubt. That's what the announcer you heard is suggesting you do. (Interestingly, Shel"ah actually also says to have those commands in mind during the mitzva blessing itself with the intention "...who commanded us regarding reading the megillah [and fulfilling its contents]" but later authorities didn't quote that part and thus it doesn't get as much announcing.)