const int y = 100;
const int x = 100;
const int offset = 20;

IList<PolygonPoint> bounds = new List<PolygonPoint>
{
    new PolygonPoint(0,0),
    new PolygonPoint(0, y),
    new PolygonPoint(x, y),
    new PolygonPoint(x, 0),
};

IList<PolygonPoint> hole = new List<PolygonPoint>
{
    new PolygonPoint(offset, offset),
    new PolygonPoint(x - offset, offset),
    new PolygonPoint(offset, y - offset),
    new PolygonPoint(x - offset, y - offset),
};

Polygon polygon = new Polygon(bounds); // here polygon contains four dots
polygon.AddHole(new Polygon(hole)); // and here - eight

P2T.Triangulate(polygon); // here I get ten triangles

foreach (var triangle in polygon.Triangles.Where(tr => tr.IsInterior)) // <-- problem
{
    // draw
}

IList<PolygonPoint> hole = new List<PolygonPoint>
{
    new PolygonPoint(offset, offset),
    new PolygonPoint(offset, y - offset),
    new PolygonPoint(x - offset, y - offset),
    new PolygonPoint(x - offset, offset),
};