Easy based upon how board feet are calulated in NHLA rules for rough sawn lumber. It's just rounding. We're going to cutting this board in half lengthwise, proper pratice gives a couple of inches to play with.
The simple calcution for 4/4 or 1" lumber is length in feet x width (measued to 1/8 inch) this gives you units(1"x12 ") then divide by 12 to turn into board footage. The trick is a board foot in non dimensional lumber is rounded to the nearest board foot.
So a board 8 foot long by 4 3/4 inches wide is 38 units(1"x12"). Divide by 12 that equals 3 and 1/6 board feet. That rounds down to 3 board feet.
If it's 1 board 4 foot long by 4 3/4 inches wide is 19 units(1"x12"), divide by 12 is 1 7/12 units, that gets rounded up to two board feet. Then add in the second piece you now have a rounded up to 4 board feet. It's the exact same area that's being measured, rounding messes it up from how we logically think.
Don't know if the same rules apply to dimensional lumber. Even if they do I can't think of an existing dimension piece that would have the issue.
I also can't say how lumber yards do it after it's been planed and ready for retail, I'm describing how it's sold.