yom kippur - Is the Book of Life ever really sealed?

In davening and in greetings, we always refer to the "sealing" of the Book of Life.
כתיבה וחתימה טובה¹ we say; בראש השנה יכתבון וביום צום כיפורים יחתמון² we pray.

Wikipedia says:

The High Holidays are times that are especially conducive to teshuva. Yom Kippur (the Day of Atonement) is a day of fasting during which judgment for the year is sealed. Therefore, Jews strive their hardest to make certain that they have performed teshuva before the end of the day.

But at the same time, it says that:

• אפילו חרב חדה מונחת על צוארו של אדם, אל ימנע עצמו מן הרחמים.³
  -Berachot 10a



• The gates of petition are sometimes closed, but the gates of repentance are always open. Bereshis Rabbah 21:6


• Repentance is compared to a sea. Just as the sea is eternally open, so too the gates of repentance are eternally open.
  -Pesikta de-Rabbi Kahana 24.


• Nothing can stand in the way of repentance.
  
  

Therefore my question is:

Assuming that repentance is always accepted, what is the meaning of the sealing of the Book of Life?

Edit: Thanks for the answers so far! But I'm still hoping I can get a source that discusses this topic directly, and not tangentially.

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^{Edit: Hat tip to Joshua for the quotes on repentance.}

^{1: [May you be] written and sealed for [a] good [year]
2: On Rosh Hashana it is written, and on Yom Kippur it is sealed, [who will live and who will die......]
3: Even if a sharp sword is placed across a person's throat, he should not despair of Mercy (To explain this as meaning that one just doesn't know what the sentence will be, and therefore should not despair, as opposed to that the mercy is granted while the sword is on one's throat, seems forced to me)}