The answer to that question is actually pretty straightforward, @Stracci. The “Sterling” and “Navajo” stamps are narrow and much smaller compared to the very large surface area of the signature stamp, so with equal striking force applied through the hammer, the smaller stamps will impress much more deeply into the metal. These stamps are applied before any other stamping or forming takes place.
Once the quality and hallmarks are applied, the silversmith flips the bracelet over into the swedge block and applies stampwork to the forward facing surfaces, stamping one face of the triangle wire, and then the other, and finally flipping the bracelet so that the peak of the triangle is fully facing forward and applying stampwork to the apex surface. Long story short, the sharp hammering pressure applied to these top surfaces flattens out the quality mark stamping on the back surface incrementally as stamps are hammered hard in to the opposing surfaces, and because the signature stamp impresses less deeply than the other quality and origin markings, the larger, much shallower signature stamp flattens out and virtually disappears in the process as it is hammered flat.
Congrats on that awesome find!