When you transform something with CTRL + T and then exit transformation and selection of the transformed object the arrows keys are still working when traveling between frames, but once you click on the timeline the arrowkeys stop working. At this moment a transform tool is active, even if you haven't used it (CTRL+T should not count as a transform tool activation as permanent). If you switch to a brush tool, the arrows keys will start to work again and you will be able normally travel horizontally on the timeline.