Strange Problem

Hi all,

I´ve got a strange problem, hope you could help. I´ve got a table (a ) with n tuples, and two fields , birthplace and birth date. There´s another table ( say b) with m tuples, where i´ve got a lot of polygons ( with their code, same as bithplace). Now i want to make a join, to obtain how many people corresponds to each place-polygon. I write

create table personsbyplace as select a.birthplacecode as code,count(*) as peoplecount from a,b where
a.birthplacecode=b.polygoncode group by birthplacecode

Well, what i can´t understand is why "Select sum(count) from personsbyplace" is > than "a" row number! (n)

In "b" i´ve got all places and "a" table is a list of people, with their place and birth date. It can´t be greater sum(count) than number of persons! Where is the mistake?
Thanks in advance,

Gus