A link to the pattern, if possible, would be helpful, even just a link to a photo of the project.
But here are my thoughts. Make real sure there isn't a dropped stitch somewhere. If you're sure there isn't, I'd leave it be cause one stitch difference isn't going to affect the size and no one will ever know you cast on 59 instead of 60.
What confuses me, though, is if you cast on 60, then eventually P2 tog, you'd have 59 stitches. If the next step doubles that up to 120 -well, as math challenged as I can be most of the time that just doesn't seem to work. That is mainly why I'd like to see the pattern.