Method injected inside header of data grid is being called for each data cell rendering.

hello , I have found that , method injected inside header of data grid is being called for each data cell rendering and also being called when we do horizontal or vertical scroll . For reproducing this issue i have attached plnkr link here . http://plnkr.co/edit/3E8HTz4Z2daGqRh1WHtx?p=preview

For instance : Below is header cell view template, we have injected {{ grid.appScope.getHeaderLetter( col.name, $parent.$index )}} and when we do horizontal or vertical scroll or at the time of initial cell rendering getHeaderLetter method is being called for each data cell. In another words if we have 100000 cells then method is being called 100000 times. I think I am missing some important fact in terms of use cases . I would highly appreciate your comment on this.

----Header Definition----

<div class="ui-grid-top-panel ui-grid-top-panel-single" id="{{'ui-grid-index-' + col.name  }}" style="text-align: center">
  <div class="ui-grid-alphbet ui-grid-alphbet-first">{{ grid.appScope.getHeaderLetter( col.name, $parent.$index ) }}
  </div>
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     more code 
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