May lose less in relative terms and more in absolute ones
Firstly, iPhones are generally rather more expensive than Android phones. If both depreciate by, say, £100, it's a higher proportion of the Android phone cost than the iPhone one. In opportunity cost terms, though, you make the same loss, having tied up a larger sum of money (or committed to a larger contract) initially. Samsung are also relatively premium products, and lost a lower proportion of value than generally cheaper makes of handset.
Secondly, durability is an additional question. Most items, if they were hard-wearing, would tend to keep their value well. However, because a tech becomes increasingly obsolete, this effect is limited. The opposite effect may dominate, that of scarcity. If phones are easily broken (and I've seen countless people using iPhones with cracked screens on the Tube), then getting a good condition example after 6 months may be much less likely, so those that have had 'one careful owner' will face much less competition on the market, leading to a higher resale price. As a result waterproofing on many Samsung phones may not improve the resale value.
Finally, the Samsung chart shows a significant drop then increase in value that seems likely to have been the result of negative media commentary surrounding the Note debacle rather than providing any useful information about iPhone vs. Android resale value trends, so it should probably have noted such substantial external factors.
Many should probably stick with somewhat less expensive phones - whose entire purchase price would be comparable to the drop in value of many iPhones. If you want the latest and can afford it, I doubt you're overly worried about the resale value anyway, even if your phone lasts that long. If I'm locked onto a 2 year contract I don't really care what the resale value is after 6 months