Anyone who has built something out of timber has had the same afternoon. You buy what the plan says, start cutting, and run out with one part still to go. The plan was not wrong. The cutting order was.
A cut list is a packing problem
You are not really working out how much wood you need. You are working out how your parts fit into the lengths the shop actually sells. Those are different sums, and only the second one tells you how many boards to put in the trolley.
The blade eats the wood
Every cut removes material. A typical blade takes about an eighth of an inch, which sounds like nothing until you make eight cuts down one board and lose a full inch to sawdust. On a tight layout that is the difference between getting four parts out of a board and getting three.
Cut the longest parts first
This is the rule that costs nothing and saves the most. Short parts will happily come out of offcuts. Long parts will not, so they need the full boards while you still have full boards.
A worked example
One 96 inch 2×4. You need two 32 inch legs, a 22 inch rail and an 8 inch brace. That is 94 inches of parts, which looks like it fits with room to spare. Add an eighth of an inch for each of the three cuts between them and you are at 94 and three eighths, leaving about an inch and a half of offcut. It fits, but only just, and only because the long parts went first.
Board feet is a volume, not a length
If you are buying hardwood you will be quoted in board feet: thickness times width times length in inches, divided by 144. It is easy to get wrong in your head and expensive to get wrong at the till, particularly on thicker stock where the multiplier bites.
LumberList is an Android app that packs a cut list into the stock you actually have, leaving a kerf between every cut and telling you how many boards it took and how much of them you wasted. Useful before a trip to the timber yard rather than after it.