That's the average bpa deviation of the previous 6 largest deviations from trend yield since 1960. Those are shown in the included graph. The largest percentage deviation in the trend came in 1972 at 15.2 percent.
While the crop conditions reported by USDA each Monday support the potential for such a record setting national average yield for corn, the two caution this year does not following the normal pattern of the other six. The normal pattern is for near or just above normal rainfall and lower than average temperatures in the three I states; Iowa, Illinois, and Indiana. However, the number one corn producing state of those three (and the nation), Iowa, had nearly twice the June rain.
"There is no historical precedent in the last five decades for an extremely high corn yield relative to trend (1972, 1979, 1982, 1985, 1986, 1987, 1994, 2004, and 2009) when Illinois, Indiana, or Iowa had such an extreme amount of precipitation during June" write the two ILLINOIS agricultural number crunchers. They add, "the same conclusion also holds when other major corn-producing states are included in the analysis".
It doesn't mean such an exception won't occur, but rather that it has not happened before. History points to record yields with cooler, wetter weather runs through August.