John,
Model 1900 Lugers have a round profile above the chamber. The 1906 model, and all thereafter, were ground with a slightly shallower, non-concentric curve. As to the reason, your guess is as good as mine.
And you are correct, the angle changes, very gently, where the angled flat portion of the receiver rails blend into the chamber. This is quite apparent if you examine the area closely with an eye to the change.
--Dwight
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