Reasons ?
There can be several reasons for this. Consider that while the natal horoscope is a snapshot in time, it is a picture of that which is constantly changing. As the Ascending degree "sinks" into the eastern horizon, the zodiac appears to rise -- you see higher degree numbers (if you could see the zodiac represented as on a computer) as time passes until the next sign appears.
This same phenomena occurs for the mathematically determined house cusps, they move forward CCW through the degrees and signs.
Astrologers utilize "orbs" or zones around a given point to show which influences are increasing (as they enter the orb-zone area or decreasing as they leave the orb-zone area). Why orbs are used and how large an orb should be is another discussion area. The astrological program most likely uses orbs to choose which house deliniation to apply to a natal chart.
Astrologers often use a combination of "orb degrees" plus a consideration of which sign is in a house and whether or not it extends into (or far into) the next house. It is often an intuitive thing as to how much orb or sign influence should be applied to a house cusp -- its not a rigid formula such as used by a computer program.
A house cusp represents a zone which has an "experience quality or activity area" associated with it through which a sign (having attitude, feeling, interest flavors) and a planet (having energy and action/idea focci) might be best expressed. The point is, unless there is a planet near the cusp or in the affected house, the influence of the house/sign combination is going to be more indirect in terms of how you see or feel or experience it.
Now, each of these statements are subject to individual empahsis and application. It is sometimes difficult to apply hard and fast rules to every chart. This is why there can be differences between what an experienced astrologer "sees" and what a computer program sees. Overall, I think they will be similar but that the astrologer will yield a more focused and specific interpretation that is useful to the querent. Hope this helps you. Dave.
