Histogram question

Lesley Jones

Otherwise know as Zooey
I'm sure the answer is in a book or on the internet somewhere, but I'm so tired I don't think a long winded explanation would sink in. Can anyone please explain why the histogram in Photoshop (I'm using CS5) isn't kept up to date? I always have a levels layer open when I work to make sure nothing is blocked or blown. Normally any changes are subtle, so I just carry on. Now I've only just noticed there is a little yellow triangle that enables you to see the histogram with uncached data - I think I've got that the right way round.

The reason I picked this up was because I've just used a gentle curves adjustment for the first time and it seemed to chop up the histogram a bit. Not too much, but I don't want tooth like spikes and I nearly deleted the layer. Then I spotted the triangle. Clicking it has given me a histogram of similar shape, but much taller and without the jagged teeth. So, am I right and the new version is the up to date one and if so... why can't it do this automatically?
 
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