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1) Google is lax at refreshing their image index. I receive several hits each day to images that have long been moved or deleted. I really should go through my log file and 301 those images to current content, but it's too low on my list of priorities.
2) Compared to Google Search, Google Image Search is a teenager. Many of its behaviors parallel Google Search as Search grew and matured. The index is getting larger so there are more images from Google to select from and they can show images from a wider range of domains. I think the days of seeing multiple images from one domain for a particular search are numbered and the results will continue to evolve and become more like the Search SERPs.