My guess is that if they are to be angled towards the sun then you have to leave enough space so that a panel at 45 degrees (to pick a number out of thin air) will not cast a shadow on the panel next to it.
Inter-module shading can be a pretty significant problem with trackers. It's surprisingly easy to hit the point where adding more panels has no benefit outside of a few hours in the very middle of the day, and for a data center Apple would probably prefer more consistent and cost-efficient power rather than higher peak power.
Wouldn't making them capable of elevating solve that problem somewhat?
When the sun is down, you need that space between the panels them to let the light hit all of them. If you elevate some of them, they will cast a shadow over the ones on the floor behind them.