sounds like a result, 3rd party insurer will know how much extra it costs when the other insurer deals with it instead of them so sounds like they are splitting the difference to save themselves a few quid as well.
Pretty much this - if they're handling it all, they have full control over the costs, if your insurer (or an accident management company are), then the 3rd party insurer have their hands tied. The £500 "incentive" to you is probably only a day's worth of the inflated hire charges they would otherwise be paying!