Why is the number of stars rounded down?
The number of stars given by the STARS tag (as commonly seen in the X-Spam-Level header) are rounded down as this header is often used for filtering. If someone set their threshold to 5 and filtered based on the presence of 5 stars, a spam with a score of 4.5 would be filtered (4.5 rounds to 5, so 5 stars would be shown)
Why isn't the message score rounded?
Scores are rounded to the nearest tenth as they are meant to be more informational in nature. This can be confusing as a message can get a score that looks like 5.0, yet still be non-spam with a threshold of 5. This is because the score is between 4.95 and 5.00. Scores in decimal are meant to be more of an informational thing rather than a filtering criteria, so there is no need to round down as with stars.
Can this be changed?
The latter has been changed in SpamAssassin 3.0; now any score less then the threshold is also reported as being smaller though this might be mathmatically incorrect.
The issue about the stars it could be changed, but in general we like it the way it is. It's a feature not a bug, so we're unlikely to fix it. However, it is relatively trivial to fix on your own if you must. The change is in lib/Mail/SpamAssassin/PerMsgStatus.pm.
See also StatusRounding for an example.