Do you ovulate before Glow predicts your fertile week?

My last green week I took some O tests and they kept being light meaning I'm not ovulating. Like they didn't get any darker. If anything they got lighter. It didn't even dawn on me that maybe I actually ovulated sooner than Glow predicted until someone said it. Does anyone else ovulate sooner? How much sooner if you do? I wanna know about when I should start testing to see if that's what is going on with me.