I hardly ever eat plain celery (but it's mostly water anyway) and I haven't noticed an issue with spinach (though again, I don't eat more than half a bowl full), so I have a feeling it's in the concentration.
I don't think it's an issue for everyone, but I know it is for some. Took me quite a while to figure it out, so I'm always hoping it might give someone an "Aha!" moment.
My sorely neglected blog - http://ThatWriterBroad.com