In some cases the CartesianGridLineAnnotation can be offset with a single pixel from the expected position on the axis. For example, if the annotation is positioned at value 0 and there is a tick, a offset between the tick and the annotation can be observed.

To work this around, you can manually offset the annotation by setting its Top Margin.

<telerik:CartesianGridLineAnnotation Margin="0 1 0 0"/>