It's a guideline simple as that. I realized how much the margins may be.
The bottom line rule on close to 95% of all the trades (there was a much larger study posted) was that 5% is the minimum a team is willing to lose in the equation. Tampa Bay is 15% above that margin.
The Dolphins are already giving a 10% advantage to Tampa Bay, you're crying about 60 points if you want to go to the 25% extreme, which was used in the case of the Lions where they wanted to select Kellen Winslow Jr, but they were offered a fantastic deal.
In the case of Philadelphia-San Fransisco where the latter was very interested in trading down it was almost an ideal trade.
Tampa Bay is in the same situation, they'd like to select Willis, Peterson (although I disagree with their logic), Okoye, or Carriker as low as possible if Johnson is gone.
Hmm fight over a late 4th rounder (extreme scenario) or save ten to fifteen million or so in guarantees and get a 10% advantage trade? I wonder...
Also Arizona at 5, is just as likely a trade up partner for Miami so that gives the latter options. The Browns did not have those options because the Redskins and the Chargers in 2004 would not budge from their selections. So this fulfills your supply and demand cries. There are more than adequate trade up partners for Miami if Cleveland passes on Quinn in some miracle. In fact three of them I'd say, not including Minnesota.
Also if Tampa Bay truly wants to take Quinn which I seriously doubt they'll take him and not even discuss the trade down. Otherwise if a nice selection like the 40 is involved they'll do the deal.