I don’t do much in blueprints, but I’m guessing it has to do with UMG DPI pixels and actual screen pixels being different. Remember that UMG scales to screen space so has slightly different coordinates.
In C++ there is “UWidgetLayoutLibrary::GetMousePositionScaledByDPI()” which we use to convert mouse cursor location to UMG HUD location. I assume blueprints have it as well.
We also use “UWidgetLayoutLibrary::ProjectWorldLocationToWidgetPosition()” in radars / minimaps.