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Purely guessing, but it might be because there's just more 5600 chips being produced than 5900 chips.

This could be due to the die size - you get less chips per wafer if the 5900 die is larger. It could also be due to yield rates - with half the cores and half the cache, it's less likely there's a flaw in the chip during the manufacturing process. Also maybe they make more profit on 5600 and prioritize manufacturing of that family for that reason.

Or maybe 5900 is just that much more popular and they want to keep OEMs happy (and fulfill their million dollar contracts) vs stocking PC hobbyists, so they push more product to the OEMs. Or likely some combination of all of the above.

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