Wood framed floors can be uneven from improper design or overload. Sometimes, this can be the result of deflection due to overstress. When this is the primary issue, structural remediation and/or reinforcement is almost always required. Much more common, especially in older homes are floors that have sagged due to creep deflection. Creep deflection is primarily a function of time, not loading. This is what is typically referred to as sag, and occurs as the house ages under its own self weight and applied loads.
We are often asked about jacking floors to level when there is a need for remediating sagging floors and other issues. First of all, it will be difficult to get a floor to perfectly level. There will always be some unevenness. Second, and most important, are the risks inherent in jacking. Finishes, electric, plumbing, etc., can all be damaged by movement of the structure which is essentially what jacking entails. There are situations where jacking may be appropriate or recommended, but the ramifications and consequences must be considered by all involved. In many cases, it may make more sense to stabilize or reinforce the floor in place and improve the levelness of it with cosmetic measures.