Cotton shrinks. That's a fact. Most tees lose 3–5% of their length and width on the first wash, especially if you dry them on high heat. Windbar tees are no exception unless the fabric is pre-shrunk. The fit chart doesn't account for that.

So if you're buying a gift, factor in shrinkage. If the chart measurements are already snug on the recipient, size up. If they're roomy, you can stick with the chart size. When in doubt, size up—it's easier to take in than let out.