The default formatter identifies when the x-data being plotted is a small range on top of a large off set. To reduce the chances that the ticklabels overlap the ticks are labeled as deltas from a fixed offset. For example:
ax.plot(np.arange(2000, 2010), range(10)) will have tick of 0-9 with an offset of +2e3. If this is not desired turn off the use of the offset on the default formatter: