I had the same thing when I done mine, I think it's because you've been playing with the piston and whne started it's gotta build pressure back up inside. I've never had to bleed my brakes after doing them and they've always worked fine afterwards.
if it were air in the lines it still would be spongey. I love doing brakes even had to redo hubby's as he forgot to make sure the spring on the brake pad was lined up:whistling lol