QModelIndex returns wrong InternalPointer - c++

Question

I have an own Qt-Model class derieved from QAbstractTableModel. By background data structure consists of Row, Column and Cell classes. So every Row has n Cells whose can bound to their belonging Column. The Cell carries the actual Grid-field-value, Background-Color, Font, etc...
My index-Method looks like this:
QModelIndex TableModel::index(int row, int column, const QModelIndex& parent) const
{
if (!hasIndex(row, column, parent))
return QModelIndex();
// modelData is my background data-structure-class
Cell* cell = modelData->Rows.at(row)->Cells.at(column); // retreiving the cell
if (cell != Q_NULLPTR)
return createIndex(row, column, cell); // Storing the Cell-object in every index
return QModelIndex();
}
Now, when the Grid accesses data I have the data-method like this:
QVariant TableModel::data(const QModelIndex& index, int role) const
{
if (!index.isValid())
return QVariant();
Cell* cell = static_cast<Cell*>(index.internalPointer()); // retreiving the Cell
// it works when I say
// Cell* cell = modelData->Rows.at(index.row()).Cells.at(index.column());
switch (role)
{
case Qt::DisplayRole:
...
// return the value
break;
case Qt::DecorationRole: return cell->Column->Icon;
case Qt::BackgroundRole: return cell->Column->BackgroundColor;
case Qt::TextAlignmentRole: return static_cast<qint32>(cell->Column->TextAlignment);
}
return QVariant();
}
My problem is, that inside the data-function the internal-pointer (static_cast<Cell*>(index.internalPointer())) always returns the same Cell (row 0, column 0) and not the Cell according to the row and column coordinate. Because of this my grid has created all rows and columns but their cell-value is always the same because it only uses the Cell from row 0 and column 0 (example below to visualize the problem).
Retreiving the Cell* via modelData->Rows.at(index.row()).Cells.at(index.column()) works though, but I really have to use the internalPointer because I would like to extend my model with hierarchy (parent-child-relations like a tree) or use the actual Cell* elsewhere.