IFRS 15

also my study text states that as per IFRS 15 the contract can be recorded as revenue when (am stating one of the conditions) it is PROBABLE that the seller will be paid.

now my doubt is shouldn’t the condition be HIGHLY PROBABLE or VIRTUALLY CERTAIN. Although later on it may turn out that the estimation of likelihood of receipt was wrong, but at least at the time of fulfilling the performance obligation the seller should be certain that he is going to receive the revenue else why would he fulfil the performance obligation in the first place?