I would believe that both subjects would have been difficult to get precise focus as there is little contrast. Although manual was used in the first picture I would wonder if the focus was really achieved at time of shot because of the pattern. With aperture selected in the second shot and focus being on the sand again there is little contrast and this could cause the focus point to be different (see page 93 in manual). I suggest you try the same procedures on subjects with more contrast and believe you'll see better results, imo. Also, I got better results with my 70-300 using an f stop around 8. Good luck.
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