Why do many people believe that the word, "thousand", in, "a thousand generations" is only a figure speech for a long time, in these three lines:

Deuteronomy 7:9 Know therefore that the LORD thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations;

I Chronicles 16:15 Be ye mindful always of his covenant; the word which he commanded to a thousand generations;

Psalms 105:8 He hath remembered his covenant for ever, the word which he commanded to a thousand generations.

But yet they believe the word, "thousand", in, "a thousand years", is definitely literal:

Revelation 20:4c  and they lived and reigned with Christ a thousand years.

  5 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.

  6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.

Why can't they both be literal?

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