jobel
Have you checked the values held in the first X and Y keyframes? You can do so by clicking on the keyframes in the timeline bar and then checking the properties bar. If those values are not 0, when the timeline starts playing, the affected instance will have a slight offset.
I am guessing you must have changed the initial keyframe with the layout view controls to adjust the starting position of the animation. Normally this works properly for all keyframes, the first one has a little gotcha if the timeline is using relative positioning though.
Because the values held are relative, by default the first X and Y keyframes have a value of 0, which just means "the initial position at the moment of playing". This allows for the animation to be played starting from the position of the instance, rather than a fixed value. If they change to something different to 0, then the animation will start from the instances position plus that offset.
Of course, this is all technical mumbo jumbo and you really shouldn't be thinking about it. I have been meaning to fix this for a while because although it is not a bug, it is a very awkward behaviour which really isn't very helpful.
Hope that helps.