Why did this go up?

According to PCGS, the 1938 P Walking Liberty Half Dollar has recently increased in value for grades VF-20 and below, while values for grades above VF-20 haven't changed. What would be the reason that only low-grades have increased? If it is due to the rise in silver, shouldn't that cause values to increase across the board?
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Don't know the reason, but since you're new here, I ask when you start a thread that you put info in the thread title so that the reader can decide if they can contribute to the thread or have the option of not opening it.
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https://www.pcgs.com/prices/detail/walking-liberty-half-dollar/733/grades-1-20
sometimes the price guide doesn't make sense
melt is ~16.50
Let's assume that a coin's price is based on a formula of (MELT VALUE) + (NUMISMATIC VALUE). The higher the NV, the less impact a change to MV has, both in terms of the actual percentage of value that comes from melt, and perhaps the more value collectors place on the coin for what it specifically is, not for its precious metal content. Similarly, perhaps as a coin becomes worth more intrinsically, the collector premium is reduced because it's seen more as a silver coin than a specific date/mintmark/grade.
Based on that, there will be coins where a change in melt value appreciably affects the overall value positively because the silver is worth more, and there will be coins where the change in metal value is perhaps negligible. And there will be coins whose numismatic value drops when the melt value increases, and the net may be no change to the total value.
Only PCGS knows why it increased the values for that range, you would have to ask them. As to your thought about the silver value, it could very well only impact the lower grades/values. While I don't track any coin/series like this (mostly because the price guides are not that accurate) it makes perfect sense that only the low grade/value items would go up if the silver value is the reason. As you go up in grades the numismatic value goes up and at some point that numismatic value is already higher than the rise in silver value.
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Probably more people can afford the lower grade pieces. But financially challenged on higher grades.
the guide increased up to and incl 20, but they left 25+ the same. this is true even for the common dates
but note the melt value, note the common coins, and ask yourself if the assigned prices even make sense