It may partly to avoid being outbid by the next 
increment (hmm... I could bid 10.51 to avoid 
being outbid, but if someone else has the same 
idea and bids 10.52 to outbid anyone who bid a 
little higher then maybe I should bid 10.53 just 
in case, but wait -- what if that's not enough?  
I guess I'd better bid 10.68!)

However, at least from my perspective, I know I 
personally have fun occasionally doing something 
random just to confuse people.  Yes, there are 
some of us out there who enjoy that sort of a 
thing. ^^