Is the Book of Life ever really sealed?

Question

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.

  Rambam, Hilchos Teshuvah 3:14 and Mishnah Berurah 128:129

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)}

Answer 1

The Gemara in Rosh Hashanah (p.16-18) has an extended discussion of these and related issues, with a few different opinions. There are different levels of gzeiros and there's a possibility of changing them after the fact. The main view there says that the gzeira for the Tzibbur is never sealed, but things do become sealed for the individual on Yom Kippur. This fits with the general emphasis in the Torah on a higher level of hashgacha for the tzibbur than for the individual (e.g. the second parsha of Sh'ma). Sincere teshuva is always accepted so that one can always change his ways and be spiritually cleansed, but that doesn't necessarily mean the course of events in this world can always change for a person at any time. However, one never knows what has been "decreed", so I don't think this view would hold one should give up just because there's a sword at one's throat.

Answer 2

I like to answer questions by undermining the assumptions, and I am going to try to do that here as well.

חתימה can mean either 'sealed' or 'signed', though they are almost certainly etymologically related. For an example of sealed, see parashat Haazinu, הֲלֹא-הוּא, כָּמֻס עִמָּדִי; {ר} חָתוּם, בְּאוֹצְרֹתָי, 'Is not this laid up in store with Me, sealed up in My treasuries?'

But a חתימה is a signature at the bottom of a document. One could argue of a get that עדי חתימה כרתי, that the witnesses to the signature are what is important. Sometimes, it is unclear which sense is being used. The חתימה of a long blessing is the end, either because it is the sealing up of it, or because it is the sign-off.

See Jastrow here and the next page.

Ultimately, it is a similar idea. Hashem is writing the gzar din, but it is not completed and signed until Yom Kippur. At that point, the gzar din goes into effect.

What about teshuva? It seems to me -- though I am not citing any sources for this -- that there is a period of judgement, where all sorts of factors about the person are evaluated, and his situation for the coming year is determined. This can shift so long as the judgement has not been signed. After that, there is a gzar din, a decree of judgement. Maybe it is harder to overcome, and one needs significantly more teshuva, for that sin, to overcome a decree. Whereas in the process of judgement, other factors (and balances) might have come into play such that the negative judgement might not have even come to be.