There are several factors governing the display of the duration of notes in the Score Editor.
A given note will have its duration truncated by a note in the same voice, if this note starts before the end of the given note.
When two or more notes start at the same time, forming a chord, all notes in the chord will have their duration equal to the longest note in the chord.
If the Extend Isolated Notes display option is set to Yes in the Display Score, Display Track or Display Bar dialog, all notes will extend in duration to the beginning of the next note, or to the beginning of the next beat, whichever is first. This makes notes appear longer than they actually are and eliminates rests. This will lead to a less exact, but less cluttered score.
To help ensure notes display exactly as entered, set Extend Isolated Notes to No in the Display Score, Display Track or Display Bar dialog.
You should also make sure your display quantization is set to a value equal to or smaller than the durations you are trying to display. Otherwise, durations smaller than the display quantization won't be displayed properly.