Then some oil must have seeped back in from somewhere..................
No. This is creating more cracks to reach farther into the formation to get to oil that was not reached in the original stimulation.
Remember these shale fields are very tight with low ability for the oil in place to flow out. Many shale wells are only getting a 3~5% of the oil in place. Hydraulic Fracturing again is trying to get more of the 95~97% of the oil that was still in the ground in the first place.