Why are my deck boards of uneven width?
I've never noticed that before, but Home Depot sources their lumber from different mills, so they may vary slightly from batch to batch.  It may also even out a bit as the wood dries.

One thing I did find, at least in my area, is that the 5/4 deck boards from Home Depot are significantly different than the ones at Lowes.  The Lowes ones are slightly narrower but a bit thicker and the depot ones are thinner but slightly wider.  Was very annoying when I got some boards from one place and more later from another only to find that they weren't really the same dimensions.