If it's a brand new Prophet 12, I'd return it and get a replacement.
It's not usual for the keys to be different levels and the Mod wheel on yours does seem to move a little more than on my P12.
If this was your only issue, I'd say it was minor and to keep it, but as you have some lower level keys too, I'd say get another unit.
The Prophet 12 is a truly wonderful synth, but it's not that immediate for a synth with over 50 knobs on it so be patient.
In the first couple of months I considered not keeping mine because of this, but trust me it'll suddenly 'click' with you and you'll wonder how you ever struggled.
I would never be parted from mine - absolutely love it!

Hope you get it sorted quickly and hope you enjoy using it half as much as I do!
