Why The tab container ActiveTabChanged event fired by the internal control changed events?

Sir,

The below is my design:

<asp:TabContainer ID="Tabcontainer" runat="server" AutoPostBack="true" ForeColor="Gray" CssClass="ajax__tab_custom" Height="100%" Width="100%" OnActiveTabChanged="Tabcontainer_ActiveTabChanged" >

<asp:TabPanel ID="Tabpnlplasma" runat="server" >
<HeaderTemplate>Tab1</HeaderTemplate>

<ContentTemplate>

<asp:DropDownList ID="d1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="d1_SelectedIndexChanged" >
<asp:ListItem></asp:ListItem>
<asp:ListItem>1</asp:ListItem>

<asp:ListItem>2</asp:ListItem>
</asp:DropDownList><br />
<asp:ListBox ID="l1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="l1_SelectedIndexChanged">
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
</asp:ListBox>
</ContentTemplate>
</asp:TabPanel>

<asp:TabPanel ID="TabPanel1" runat="server">
<HeaderTemplate>Tab2</HeaderTemplate>

<ContentTemplate>

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem></asp:ListItem>
<asp:ListItem>1</asp:ListItem>

<asp:ListItem>2</asp:ListItem>
</asp:DropDownList><br />
<asp:ListBox ID="ListBox1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged">
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
</asp:ListBox>
</ContentTemplate>
</asp:TabPanel>
</asp:TabContainer>

Here my problem is when i select the Tab1 dropdown or listbox two events are fire one is dropdown or listbox and another is ' tabcontainer active tab changed event'.

When i repeat this same process in tab2 the Tab container event not fired. Why this tab container event fired . please help me sir.