I suspect the statistics graph is actually just incorrect. It uses timing to calculate the rate. Hashing the newly completed file is a very CPU-intensive operation. I suspect it's somehow preventing aMule from keeping up with its housekeeping tasks for a brief time, and as a consequence aMule is getting confused about how much was uploaded in a particular period of time.
Use Mac OS X's Activity Monitor to monitor the Network Activity to see if it really spikes like that.