Another thing... Screen printing is done with thick inks, almost like paste. The process requires the ink to be diluted, to achieve a really specific viscosity. A constant room temperature and humidity is important. Viscosity will also change during the process as the ink will evaporate a bit, over time.
I think that's why we see different opacity amongst pictures. Viscosity changes. Thin ink, shadowy text. Thick ink, opaque text.
I'm no expert, that's only my hypothesis.