I've been thinking about this since my Rey and protection gains post earlier. Here's what my best guess is:
Max Health gains will always "give" health. Example: Nihilus annihilates and gains 50k Max Health will also give him 50k health points.
Max Health decreases take away from the top but won't take health away unless the amount being subtracted reaches your current health.
Examples: Nest has 80k Max Health but has 75k current health. Her Max Health is decreased by 10k so her Max Health goes to 70k. Since her current health pool is more than that, she loses 5k bringing her to 70k or 100% health.
Think of a water bottle. If I have a bottle with 80% water and I take a katana and slice it half, I will lose some water. If I just take the top off, it's "filled" percentage is closer to 100%. It may look like you gained water (going from yellow to green health) but the percentage just changed, not the amount.
Again... my best guess. Who really knows.