In my opinion some of the older versions of the vertical flutes has a tube that decreases in size from tope to bottom, much like many of them still do today. Another reason is that the flute music gives a somewhat calm and relax feelings to the hearer. This might be also caused by the wood-like sounds native flute produces.
I am learning to play flute. The flute is a piece of wood with a lengthways hole bored through it. Playing in tune is my problem on any types of flutes. I am trying to learn Pearl flutes and Jupiter flutes.
You can find last year that is also applying more pressure to fniger already low. If so, try the same thing but without pressing the string. The two finger on the trill should barely touch the string, but do not push.