The renderer will override keyframes that are on the 'Compositing' checkbox (Post processing panel of Render tab) based on whatever value it is at the time of rendering. The easiest way to show this is to animate the checkbox to be off then on.

The attached file has a keyframe on frame 4 setting the 'Compositing' checkbox to be off. On frame 5 it is set to on. To show the effect of compositing a simple mix node has been added to the compositor turning the render red.

To replicate problem (using attached blend file):

1. Place the timeline playhead on frame 5 (on frame 5 a keyframe sets the compositor to on)
2. Render frame 1 to 1 by clicking on 'Render animation' button in the Render panel in the Render tab. (at frame 1 compositor *should* be off)
3. Result shows that compositing is still happening on frame 1 even though the keyframe to turn it on doesn't happen till frame 5.
4. Place the timeline playhead on frame 4 (on frame 4 a keyframe sets the compositor to off)
5. Rendering frame 1 to 1 again shows compositing is not happening.

If you were to render from frame 1 to 10 when the playhead was at 5 then compositing would be on for the whole animation regardless of keyframes. If the playhead was at frame 4, when rendering the same thing, then the whole animation would have compositing turned off.

The effect can't be seen with still frames as rendering a still will only render the frame the playhead is on (and would then read the accurate value for the frame).

This seems to show that the value of the checkbox is read as whatever value it is at render time, regardless of any keyframes which should change it.