building a kernel from a mounted partition

Hello, I have a dual boot machine, Ubuntu/Fedora. I have a kernel tree on the fedora partition. Now, I had build a kernel on the Fedora partition successfully. I did **not** change anything in the tree, and then When I boot into Ubuntu and mount the fedora partition, and go there and run "make" , it builds ** all ** the tree again ! any I idea why ? And after the build on Ubuntu is over successfully, if I boot into Fedora again and run make, it **again** builds all the tree. Is there a way to avoid this ? rgs Andy