@Tom R. I have worked in paint sales for 10+ years and spent 3 years in residential painting. I don't work for Home Depot, but I can certainly speak on the behalf of "the other side of the counter" on this one. Matching paint is an art, not a science. Some matches turn out perfect, some don't. I have seen countless matches look dead on in the store and not match in the field. Often times a small sample brought to the counter won't tell the whole story of sheen/gloss, color loss due to UV exposure, lighting at the jobsite, etc. The more comprehensive the sample, the better the match tends to be.
I have matched thousands of gallons to every type of sample imaginable (bags of dirt, fabrics, photos, descriptions, food items, etc) and I would never place a "money back guarantee" on my skill set. I would however, always be happy to make changes to the match to satisfy the purchase. A little light? Great, I will add a small measurement of black pigment at no charge and continue to do so based on your feedback/photos/additional info.
The folks at your paint store can be a great asset to you if you treat them like members of your team. They can connect you with other investors, subcontractors from many trades, and even provide creative leads (build a relationship with your local field sales reps here).
Feel free to reach out if you have any paint related questions!