Why lookup in SortedDictionary<> is SLOWER than lookup in Dictionary<>?

As as human I always thought that lookup in something sorted is the way faster than loo开发者_如何学JAVAkup in not sorted.

But looking at this http://dotnetperls.com/sorteddictionary I can say that I was wrong.

Maybe anyone can explain why it is so ?

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The unsorted dictionary is probably a hash map so lookup is almost O(1) assuming not too many collisions, while a lookup in a sorted list is best case O(log N)