Trim/Crop Your Artwork
When you check the measurements of your artwork, you will need to measure it from the edge of the artwork to the other edge of the artwork. You would not measure the background edges, as this is probably not a part of your print.
You can manually use the crop tool and crop all 4 sides of you artwork to gauge the true size. If you have snapping enabled, the crop box will automatically snap to each edge. Some rip software will allow you to crop from within, but it's better to have your artwork as prepared as possible beforehand, and let the rip just handing printing.
Or for a more advanced way, use the Trim option located under the image menu. After selecting Trim... a box pops up. You can "trim" based on either transparent pixels (no background, appears with grey/white checkered pattern behind art) or a color. Many times black or white, which happens a lot, or to represent the garment color. Once you do this, Photoshop trims away the blank areas, leaving your artwork at the very edge of all 4 sides.