Balsa is not a constant. It’s like buying produce. There is a lot of variance and this leads to a higher waste and labor content when using it in you projects.
For instance I was looking at a batch from a new supplier and I suspect they picked the batch over for a specific weight (9-12lb) and then sold me the rest. So I took a sample and weight them to make a histogram of density. I got the figure at the right. I expect a normal distribution for balsa density (perhaps with a longer tail to the heavy side). There could be a double peak in the histogram proving my point. But another good explanation is that the shipment came from two sources.
Sometimes you also deal with uneven cutting. As you can see below the bundle of wood is not square.
This is a challenging aspect of dealing with a product of nature.