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Alignment, Size, and Type: While a monster's size and type remain constant (unless changed by the application of templates or other unusual modifiers), alignment is far more fluid. The alignments listed for each monster in this book represent the norm for those monsters—they can vary as you require them to in order to serve the needs of your campaign. Only in the case of relatively unintelligent monsters (creatures with an Intelligence of 2 or lower are almost never anything other than neutral) and planar monsters (outsiders with alignments other than those listed are unusual and typically outcasts from their kind) is the listed alignment relatively unchangeable.
This sums up all of those phrases - the GM determines what the alignment of a monster is, the rulebok gives the norm for the monster (world-neutral that is, so even that might change).
There's no more often / usually / always designations on alignment. Unintelligent creatures and aligned outsiders are pretty close to always of their listed alignment. Everything else varies by campaign (as per feytharn's earlier quote).
IIRC unofficial statements also indicate that undead have a pretty strong but not unbreakable bias to evil (Blood of the Night talks about nonevil vampires).