Fred58,
Your example is a correct, if extreme situation. Basic statistics says that the standard deviation, i.e. the approximate width of the spread of values in a sum, grows proportionally to the square root of the number of entries summed. So with a 100 entries, you would expect a 10-fold growth in the range of possible swings away from the penny-precise average. (I ignore the technicality that the +/- 50 cent penny range is a uniform distribution and not a normal (bell-shaped) distribution.) For 100 entries, 95% of the time the swing will fall within about $5.75 each way. $18 is an outlier, but, as you have seen, not impossible.
