Quick Overview of Dynamic Needs
So, rather than always requiring the same amount of water, food, and power, those numbers can change based on a number of factors.
Things like the level of your buildings, the total number of plots (of all different types) you have (in that settlement), the number of stores, even the VIP's can affect it - essentially everything you build in Sim Settlements can have little effects on the needs that your settlement has.
As you build out your settlements your settlers will respond to what you build and how you manage their settlement, and as a result their needs will change over time as their settlement grows - hence Dynamic Needs.
The 999 Problem
If a settlements resources are at 999, it means that the resources are being used into a deficit. The vanilla Fallout 4 settlement system cannot handle negative numbers, so the resource count jumps to 999. This can be due to Dynamic Needs being on. With dynamic needs, some structures will require more Power and Water to run and may reduce other settlement numbers such as Food or Defense.
TODO: Explanation of how the Dynamic Needs system works and how to check needs with the HUDFramework HUD or in the holotape via the Tools menu.