This creates an interesting situation. The Umpires made the correct decision between them, but under the DRS, their decision was in error. This is due to a condition in the DRS specification which requires that the consultation between the TV Umpire and the on-field Umpire must be about points of fact. The TV Umpire is prohibited from saying whether or not the On-field Umpire made a correct decision. His role is limited to providing the On-field Umpire with "facts".
j) The on-field umpire must then make his decision based on those factual questions that were answered by the third umpire, any other factual information offered by the third umpire and his recollection and opinion of the original incident.
k) The on-field umpire will reverse his decision if the nature of the supplementary information received from the third umpire leads him to conclude that his original decision was incorrect.What were the facts supporting the TV Umpire's conclusions? He does get a high standard of audio and video information. Given that the decision was reversed, it must be assumed that conclusive evidence was conveyed to the On-field Umpire to reverse his decision. How can we reconcile this with the fact that the heat signature registered nothing conclusive? Umpire Davis, in effect made a marginal call. The correctness of his decision was later confirmed by Dravid himself. But was it correct within the remit of the DRS? As Subash Jayaraman points out here, there is a difference between Umpire Davis's role in an Umpire Review (as in the case of Suresh Raina's dismissal on third evening at the Oval) and his role in a Player Review. The Player Review involves reversing a decision, while the Umpire Review involves a situation where no decision has been made.
But here is where the DRS's problematic nature is illustrated. Far from serving the original purpose of solving howlers, the DRS has tried to eliminate marginality. The fact is, that because the technology has its own limitations, it introduces new areas of marginality to the situation. Hotspot doesn't always work. It works differently when it is sunny and when it is cloudy. This, in and of itself, is not a problem. It would be a mistake to make the perfect the enemy of the good in the case of DRS.
But the DRS's protocol needs significant review. While Umpires clearly possess the expertise to exercise specific judgments, the protocol, as it is currently written prevents them from doing so. The protocol has to make better use of the Umpire's judgment, and hopefully, some day, the ICC will see good sense and eliminate the Player Review and replace it with a pro-active 3rd Umpire Review.
DRS, as it currently stands, places the Umpire in an impossible position, because it prevents them from making the sort of judgment that Umpire Davis appears to have made. Marginality cannot be avoided. And marginality necessary requires inconsistency in rulings. It will be much better if the ICC's DRS accept this inconsistency by allowing the TV Umpire greater leeway when reviewing an Umpire's on-field decision.