When you say "in lower light", do you mean indoor, by tungsten?
If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green, oups!
If you are familiar with lenses, you'll see that some have a little red mark under the distance scale, slightly off the main mark. See below, right between 4 and 11.
This is the focus indicator for infrared, and shows quite prettily the impact of wavelength on focus.
Anyway, a body focusing perfectly outdoor will be a little off indoor.