Why is Vera referred to as "a very self-possessed young lady" in "The Open Window"?

Saki emphasizes that Vera is
"self-possessed."

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"My aunt will be down presently, Mr. Nuttel,"
said a very self-possessed young lady of fifteen; "in the meantime you must try and put
up with me."

Here the child's voice lost its self-possessed
note and became falteringly
human.

Vera is calm, cool,
poised, relaxed and sophisticated for her age. The main purpose for presenting the
character this way is for the sake of contrast. When the three hunters appear outside
the house, all heading towards the open window with their guns, Vera's apparent loss of
her self-possession is all the more effective. It is also believable, because we know
that Vera is not really as self-possessed as she is trying to
appear. 

The
child was staring out through the open window with a dazed horror in her
eyes.

This goggle-eyed,
open-mouthed look of "dazed horror" convinces Framton that these are the very same three
men who were supposedly killed three years ago.

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"In crossing the moor to their favourite
snipe-shooting ground they were all three engulfed in a treacherous piece of
bog."

They are the same three
men that Vera's rattle-brained aunt has supposedly been expecting to return for tea for
all those years. It is the look of horror on the heretofore self-possessed young girl's
face that communicates fear to Framton and makes him flee for his
life.

Vera can talk about the men being engulfed in a bog
without showing much emotion, because she was not there when it happened and claims to
know only what her aunt told her.

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"Poor dear aunt, she has often told me how they
went out..."

But when the
three hunters actually appear outside, Vera can act incredulous and horrified for
Framton's benefit. The girl knew she was going to have to fake such a look when the time
came. Maybe she had already practiced it in the mirror and was just waiting for the
proper opportunity to arrive.