FORTUNE — Mitt Romney has been a politician for the past 13 years, but his sales pitch to voters relies heavily upon his private sector experience. Sometimes, however, the juxtaposition doesn’t ring true.

For example, check out this exchange from Tuesday night’s presidential debate:

Crowley: “If somehow when you get in there, there isn’t enough tax revenue coming in, if somehow the numbers don’t add up, would you be willing to look again at a 20 percent…”

Romney: “Well, of course they add up. I was — I was someone who ran businesses for 25 years and balanced the budget.”

I totally understand why Romney said what he said from a campaign context, but it doesn’t make a whole lot of sense.

As any venture capital or private equity investor will tell you, there is a big difference between what you think you know from due diligence and what you actually learn once buying the company. Moreover, the ability to lithely adjust to changing circumstances — sometimes internal, sometimes external — is a vital private equity trait (and one, many would argue that Romney has applied to his political career).

Mitt Romney the private equity executive would never tell his investors or colleagues that the pre-deal math was a slam dunk to add up. So Mitt Romney the presidential candidate shouldn’t tell voters that pre-election math is a slam-dunk, particularly if he then is going to use his business background to legitimize the assertion.

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