Your simulated far field directivity gain (at 5.1GHz, where your simulated S11 = -15dB) is much lower than in the paper (4.6dBi vs about 11dBi)
So the diference between your S11 vs frequency and paper S11 may not be only the feeds.
Could be also differences related to array characteristics, especially mutual coupling between elements or substrate differences, characteristics that may affect the return loss of each antenna element.
However, due to changes in surface waves, those array types using grounded elements, gets higher mutual coupling between antenna elements.