This is because variable reinforcement schedules are less predictable than fixed reinforcement schedules. There are two possible types of variable reinforcement schedules: <em>variable-ratio</em> or <em>variable-interval.</em>
Variable-ratio reinforcement schedule means that the reinforcement would only be given when the individual exhibits the desired behavior <em>according to a number of responses</em> – these numbers are unpredictable for the subject. Variable-interval uses the same approach, however, the unpredictability lies <em>on the period of time from when the desired behavior appears to when the reinforcement would be given</em>.
This unpredictability causes extinction to be harder for the reinforced behavior since the subject might think that the reward for their behavior might still be given.