The core of confusion is probably that radar data is buried in a lot of noise. That would be the case if the aircraft was flying low, or if it was approaching the limit of radar range, or if the radar is not in a perfect health. They probably have only disconnected groups of dots. If that is the case, they would have difficult time being sure about continuity of any unidentified dots, especially in the presence of many other signals and general electronic noise in the area. The only sure way would be to compare data, possibly even raw radar data of all neighboring counties. However, that would reveal real, practical and not theoretical limits of their radar detection capabilities, and information about early warning capabilities is absolutely crucial for any military. Most likely, that is core problem now, they do not know how to safely cooperate at this extremely sensitive and highly technical level.