I have a large stag-horn fern . This spring after moving it out side ,I found it heavily infested with scale. I choose to scrape them off with my fingernails rather than use oil spray or wait for a systemic to work. Two weeks ago i scraped off about 300 scale over 2 days. Today I checked it expecting to find some ,like 20 ? There were more like 60 but they smaller like 1 to 2 mm. Original scale was about 1 to 4 mm. So is it more likely that smaller scale reattached or the tiny scale grew in 2 weeks ? When I scrape them off ,some are wiped away and some debris stays on the frond.
Probably what you scraped off were adult scales - crawlers may have been beneath, or were too small to notice and you only saw them once they got a bit bigger and attached themselves. New scale insects are unlikely to be the old ones reattaching themselves. Staghorn fern can be very sensitive to insecticides, and often insecticides aren't that effective on scale insect, so I'd recommend you carry on checking and scraping them off. https://homeguides.sfgate.com/scale-insects-staghorn-ferns-28722.html