7+7=18.6
Got it.
Seven years fat followed by seven years of lean doesn't necessarily equal 14 years as measured on the Gregorian calendar.
There's no reason to think they meant that the fat and lean periods were exactly seven years, following each other with metronomic precision.
Further, the ancient Iraelis used a 13 month lunar calender.
If the good professor is correct about his lunar cycles, it would make sense that civilizations which depended on lunar calendars would have observed this, as well.
Include some transitional time and yeah, it might work out.