Bug in a Scroll of DataGrid - WPF? -

i hope can me this:

i have datagrid 20 rows , every time check checkbox in 1 of columns, label value increasing 100 each time or decreasing if unchecked. so, once i've checked rows value of label 2000. see below:

when reduce height of datagrid vertical scroll appear , when i'm checking checkboxes procedure seem similar can see:

but if scroll vertical bar downwards, other checkboxes appear enabled or checked, idea why? in case i've checked first 6 checkboxes , value of label 600, fine, why other ones appear checked ? thanks..

here code i've used:

<stackpanel name="spuno"  width="auto " lostkeyboardfocus="spuno_lostkeyboardfocus" >         <button name="btn_accion" margin="5" width="100" height="30" content="accion" click="btn_accion_click">         </button>         <label name="lbl_muestra" width="300" margin="5"/>         <datagrid name="dg_pruebas" autogeneratecolumns="false" datacontext="{binding}" height="200" width="300"                  selectionchanged="dg_pruebas_selectionchanged">             <datagrid.columns>                 <datagridtextcolumn header="id" binding="{binding path=id}"></datagridtextcolumn>                 <datagridtextcolumn header="nombre" binding="{binding path=nombre}" ></datagridtextcolumn>                 <!--<datagridcheckboxcolumn header="check" binding="{binding path=ischecked}" ></datagridcheckboxcolumn>-->                 <datagridtemplatecolumn>                     <datagridtemplatecolumn.header >                             <textblock>ver                              </textblock>                     </datagridtemplatecolumn.header>                     <datagridtemplatecolumn.celltemplate>                         <datatemplate>                             <checkbox name="ck_prueba" visibility="{binding path=visibility}" checked="ck_prueba_checked" unchecked="ck_prueba_checked" ></checkbox>                         </datatemplate>                     </datagridtemplatecolumn.celltemplate>                 </datagridtemplatecolumn>             </datagrid.columns>         </datagrid>   private void ck_prueba_checked(object sender, routedeventargs e)     {         checkbox check = e.originalsource checkbox;         objeto obj = dg_pruebas.selecteditem objeto;         if (obj == null)             return;         if (check.ischecked.value)          {             lbl_muestra.content = (convert.toint32(lbl_muestra.content)+convert.toint32(obj.valor)).tostring();         } else          {             lbl_muestra.content = (convert.toint32(lbl_muestra.content) - convert.toint32(obj.valor)).tostring();         }      } 

it recycling speed
try

virtualizingstackpanel.virtualizationmode="standard"