I don't quite understand why, myself, but that is what the surgical books say.

When anybody is hurt and bleeding badly, the first thing to do is to find out whether it is an artery or a vein that's cut.

If the blood is bright and comes out in spurts, it's an artery.

If it is dark, and flows steadily, it's a vein.
If it's an artery and isn't cut quite in two, tear it in two.

If that don't stop it, you must make a knot in a handkerchief and then press your finger above the cut in different places till you find where the artery is by the blood stopping.