The reason for this is the nearer term option that you are selling is then out of the money which reduces the initial probability that the option will be exercised by a counter party and you will have the option assigned.

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The primary reason is that the probability of an option being out of the money on expiration day (Prob OTM in the graphic above) is 58% in this example.

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What Beach has effectively done is sell Chevron an out-of-the-money option.

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One other quick point: you may want to start with an out of the money call butterfly or put butterfly in order to lessen the probability of another party exercising an option you sell, as it is detrimental for them to do so while the option is out of the money.

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