It seems to me that the GNSS postprocess software gives baselines as if measuring by sight or a laser. So If a line is projected "east" level to the surface it will continue to elevate with respect to the earth because of its curvature. When it reaches the target point it will be above the original ellipsoidal height. This is why the baseline up value is 600 for PUNT-CASA baseline while the difference in ellipsoidal height is much larger.
Not sure if this is new info for you? How do the other type of baseline work?