There is no reason whatsoever to run the internal grabber more than once a day, shortly after your wget is completed. If you get the file at 2 AM then try something like
15 2 *
To run the internal grabber at 2:15 AM.
But still, the line you used shouldn't be causing that problem since as you note it is the default setting. I'd try changing it just to see what happens, but if that doesn't help then I'd wonder what else might be going on. I have never seen that happen, but I have the default commented out and only run the grabber once a day. You didn't say what version you are running but if it's a nightly, you may have uncovered a bug!
EDIT: Also, I'd make sure you don't accidentally have any OTA grabbers enabled.