Maybe the ones on pizza’s are more dry because of being in the oven, so that there isn’t that much juice in them (the bigger the combination of juice and salt, the less salty taste you’ll get?). The ones you eat from a tin are kept in oil, or am i mistaken?
Some anchovies are cured and packed in salt, and need to be soaked. Some are packed in oil, usually olive oil. The tinned grocery-store anchovies are usually packed in oil. Bulk anchovies are usually salt-pack.