Pierre-Antoine,
I am sorry to say that I could not recreate your problem. I tested by change the HowDoI sample TrackAndEditShape sample; the following code is its loading code with accound and password needed.
winformsMap1.MapUnit = GeographyUnit.Meter;
winformsMap1.BackgroundOverlay.BackgroundBrush = new GeoSolidBrush(GeoColor.GeographicColors.ShallowOcean);
string accountId = "****";
string password = "****";
string clientIpAddress = "66.249.71.109";
string cacheDirectory = @"c:\temp\";
VirtualEarthLayer velayer = new VirtualEarthLayer(accountId, password, clientIpAddress, cacheDirectory);
velayer.MapType = VirtualEarthMapType.AerialWithLabels;
LayerOverlay layerOverlay = new LayerOverlay();
layerOverlay.Layers.Add(velayer);
winformsMap1.Overlays.Add(layerOverlay);
winformsMap1.CurrentExtent = new RectangleShape(-143.4, 109.3, 116.7, -76.3);
winformsMap1.Refresh();
Following is three screenshot with zoomin and zoomout.
So, in your case, just make sure 2 things:
1) Did you turn off the ZoomLevelSnappingMode from default to none?
2) How did you generate your inmemoryLayer data, I means the polygon in your screenshot? I did by track a shape along some building, in this way, the tracked out polygon should be in the same projection with the VE. I am not sure your data source from, so I cannot make sure about this point.
If you still have problem it would be nice and helpful if you could make a small sample application for me.
Thanks.
Yale