Select first N records for each group, how does it work?

Question

SQL

Faced with the classical task:
In the table one field - groups, another - values. You must select, say, three values ​​for each group. It is necessary to implement in Access.
There is a working query on google like this:

SELECT Группа, Значение
FROM Таблица
WHERE Значение IN
   (SELECT TOP 3 Значение
    FROM Таблица as Temp
    WHERE Temp.Группа= Таблица.Группа
    ORDER By Значение DESC)

The result is adequate, but I can't figure out how it works. According to the logic of things, the nested query returns 3 rows, which means that it can return no more than three Values. But at the output, I get the entire set of groups with a variety of values ​​\u200b\u200bgreater than three.
Please explain the math behind the query.