Width of grid columns on different fixed pages

Question

I have a structure of items like this for example:

<FixedDocument Grid.IsSharedSizeScope="True">
   <PageContent>
      <FixedPage>
         <Grid Width="500" Height="200">
            <Grid.ColumnDefinitions>
               <ColumnDefinition Width="100" SharedSizeGroup="A"/>
               <ColumnDefinition/>
            </Grid.ColumnDefinitions>
         </Grid>
      </FixedPage>
   </PageContent>
   <PageContent>
      <FixedPage>
         <Grid Width="500" Height="200">
            <Grid.ColumnDefinitions>
               <ColumnDefinition SharedSizeGroup="A"/>
               <ColumnDefinition/>
            </Grid.ColumnDefinitions>
         </Grid>
      </FixedPage>
   </PageContent>
</FixedDocument>

Why doesn't width of first column of grids have a same value on different fixed pages?