@von Bordwehr
I can conclude that this problem must indeed be a bug (not a feature), because some text selection results cannot be of usage to anyone. Unfortuantely it affects all programs in the entire iWork suite, but at least seems to be contained in there. I therefore rule out the likelyhood of a Mavericks bug.
Take for ex a related situation to the one you describe above: "Another selection issue", Cursor before the word "selection", then Shift+Alt + righ arrow followed by Shift+left arrow. The result doesn't even include the word "selection" anymore.
This odd behavior seems to affect most text selection involving Shift+Alt+arrow keys followed by Shift+arrow keys (especially if the arrow keys are opposite). If combined also with the Command key the problem will be even greater.
I would guess it's the index of right-selection border that the iWork apps calculate wrong when the Alt key is used. In most cases it is simply mixed up with the index of the left-selection border.
I can also answer your question above (regarding what's happened) with the positive news that Apple's iWork team is indeed aware of this bug and is even working to fix it right now. First-hand info
😉