I took Alg. 1 in my freshman year, Geometry as a sophomore, Alg 2. as a junior, and Alg 3 as a senior where I reached my mathematical limit and somehow managed a D and that was the only D I was somewhat proud of.
This happens to me a lot especially when I'm working with the little kids. I just adjust my timing accordingly because it's easy to call these too quickly.
Sidebar When I was just starting I was calling pitches early because I was anxious and called them before I could doubt myself. Now I'm more growing more confident but still working on it. I doubt this is what caused your problem as you have way more experience but I'm just describing what caused my timing issues.
Because if you called it a strike you are saying it went through the strike zone therefor the swing is irrelevant whereas if it was outside the zone the swing could be the difference between strike and ball.