If a line item has more than one creative of the same size and you select the optimized rotation option, DoubleClick for Publishers will give roughly 75% of the impressions to the creative with the highest clickthrough rate, and the remaining 25% to the other creatives.
This continually-updated rotation is calculated after thousands of impressions have been served. If a creative that originally received only 25% of impressions starts to earn a higher CTR, the weighted delivery will shift in favor of that creative.
(Learn how to set creative rotations.)