Julian/Gregorian conversion goof? If it's off by about a year, especially if in January-March, this is a common reason.
Until October 1752, England and its Colonies stuck to the obsolete Julian calendar, which began the year on March 25. (The rest of Western Europe adopted the Gregorian reform starting in 1582, when it was first proposed - some of them had already made the changeover to a January 1 start to the year.)
The Colonies sometimes - inconsistently - recorded the discrepancy as a slashed year, e.g. 1725/26. This is still used sometimes when people want to be precisely accurate. But Geni does not support it.
It is also possible that the birth dates of their first child or two are rough guesstimates. There seems to be a lot more uncertainty about him and his wife than appears on the surface - I'm getting conflicting information from multiple sources (she was a Green, a Van Cleve, a Lowrance; they were married in New Jersey, in North Carolina; they were married 4 September 1749, c. 1750, 4 September 1751; etc.).
Only someone with managerial privileges can get at the Relationships field to change anything in it. I do not have such privileges, so cannot help.
This link doesn't directly address the issue, but does provide a great deal of background: http://www.tamu.edu/faculty/ccbn/dewitt/mckstmerjersey.htm