The Gemora lists a number of diverse cases where the Sages applied the rule for the robbery victim, without clarifying in exactly what circumstances the rule was applied, nor why. There is discussion in the Rishonim about what the parameters for these cases are. Rabbeinu Tam (Tosfos 62a asu) states that the debate in the case of an informer is only when the informer claims with certainty that he did not cause as large a loss as the victim claims. However, when the informer is uncertain, then there’s no question that the victim can use the rule of the robbery victim, and collect with an oath. The Ri, on the other hand, states that the Gemora is currently following the discussion of applying the robbery victim rule to fire damages of embedded items. In that case, the damager obviously has no knowledge how much he damaged, so similarly the discussion by an informer must be also in the case of the informer not knowing how much damage he caused.
According to the Ri, the application of takanas nigzal is only due to the fact that the damager doesn’t know how much the damage is. If the damager claims with certainty a lesser amount, the usual procedure must be followed, and the victim must bring proofs. The Gemora is only using the case of nigzal as a borrowed term, to apply in a case where the damager cannot counter claim with certainty. However, Rabbeinu Tam applies the takanas nigzal even to cases where the damager claims a lesser amount with certainty. The application must be a more direct analogy to the robbery case. In the robbery case, the robber is not able to swear, since we punish his status as a robber by invalidating his oath. Therefore, the Sages placed that oath on the victim, to allow him to collect. Similarly, Rabbeinu Tam holds that one who lights a fire – a gross negligence, and a very direct form of damage – and an informer – a very severe and dangerous form of damage – are punished for their crime by allowing the victim to collect with an oath. Rabbeinu Tam would therefore equally apply the takanas nigzal to any fire damages, even in the simple non tamun case that the Chachamim discuss. [See the Rosh paragraph 16, who mentions both reasons by the case of fire.]
The Pnei Yehoshua points out that the Gemora flow seems to indicate Rabbeinu Tam’s approach is correct. The Gemora concludes the discussion of takanas nigzal with a seeming non sequitur – the distinction between a chamsan and gazlan. The Pnei Yehoshua explains that the Gemora was discussing different types of criminals, and the sanctions put on them by takanas nigzal, and therefore concluded with a statement about two types of criminals who cause another person monetary loss.