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The difference in probability of exceedence and probability of occurrence can
be understood this way..

Suppose you throw two dice, the probability of getting an exact seven, no more,
no less is the probability of occurrence of seven.  the probability of 
exceedence will be the probability of getting anything greater than an exact

In earthquake ground motions and floods the probability of exceedence therefore
means the chance of exceeding a certain ground motion or flood level.  So if
you are designing a structure for acceleration level of 0.5g you want to know 
the chance that the value could be higher than 0.5g to understand what risk 
you are taking.  Who cares what the probability for getting an exact no more
no less 0.5g. (maybe in some isolated cases you may want to know that for some
forensic studies).

hope this helps
ahmed nisar
dames & moore 
san francisco