Run the enclosed program and look at the column headers. Even though the resize policy is CONSTRAINED_RESIZE_POLICY, there is extra space to the right of the last column (B). Resizing the window eventually gets rid of the extra space. However the B column can have a little bit more or less space depending whether you're making the enclosing window bigger or smaller.

import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.scene.Scene;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeTableColumn;
import javafx.scene.control.TreeTableView;
import javafx.stage.Stage;

public class TableBug extends Application {
public static void main(String[] args) {
Application.launch(args);
}

public void start(Stage stage) throws Exception {
TreeTableColumn<String, String> col1 = new TreeTableColumn<>("Resizable 1");
col1.setPrefWidth(200);
col1.setCellValueFactory(cdf -> new SimpleStringProperty(cdf.getValue().getValue()));
TreeTableColumn<String, String> col2 = new TreeTableColumn<>("Resizable 2");
col2.setPrefWidth(200);
col2.setCellValueFactory(cdf -> new SimpleStringProperty(cdf.getValue().getValue()));
TreeTableColumn<String, String> col3 = new TreeTableColumn<>("A");
col3.setPrefWidth(30);
col3.setMaxWidth(30);
col3.setMinWidth(30);
col3.setCellValueFactory(cdf -> new SimpleStringProperty("#"));
TreeTableColumn <String, String> col4 = new TreeTableColumn<>("B");
col4.setPrefWidth(30);
col4.setMaxWidth(30);
col4.setMinWidth(30);
col4.setCellValueFactory(cdf -> new SimpleStringProperty("%"));
TreeItem<String> root = new TreeItem<>("Root");
TreeTableView<String> tv = new TreeTableView<>(root);
tv.getColumns().addAll(col1, col2, col3, col4);
tv.setColumnResizePolicy(TreeTableView.CONSTRAINED_RESIZE_POLICY);
tv.setMaxWidth(Double.MAX_VALUE);
tv.setMaxHeight(Double.MAX_VALUE);
root.getChildren().add(new TreeItem<>("AAAA"));
root.getChildren().add(new TreeItem<>("BBBB"));

Scene scene = new Scene(tv, 800, 600);
stage.setScene(scene);
stage.show();
}
}