Base salary isn't only what Harper is making so that's a partial picture.
There's a reason we can't cut him with any benefit. Because he's making a ton of prorated money this year.
Harper's deal is still top 5 for a Safety in the past 2 years. We overpaid on the guaranteed money and we have no flexibility with him because of it.
While I understand what you are saying and I agree... it does seem almost like you're referring to money he has already earned via signing bonus, etc as money he is guaranteed to earn this coming year.
The majority of the time, the prorated money is usually the result of a big signing bonus which the player has already received. I believe Roman's signing bonus was in the neighborhood of $7-8 million, iirc. Spread that out amongst the 4 years and you get $1.75mil added to the cap hit annually until his contract is up, but it is not money owed to him in these years. All that he will be "making" this year is his base salary and probably some workout incentives.