There’s no real reason for them to be the same, because they don’t use the same protocol for the two methods of communication. Router to camera is WiFi, and sync module to camera is low frequency radio, From a design point of view, they would aim to ensure that both had equal range, around 100 feet, in ideal conditions, but things are rarely ideal.
For example, if the signals pass through a brick wall, or several, then the WiFi signal at 2.4 GHz would be attenuated more than the low frequency radio, which I believe operates at 900 MHz. Ultra low frequency radio, is used to communicate with submarines, at a few Hz, as an example of how lower frequencies can penetrate further.
Not saying there isn’t something odd going on, because I’ve read several times now, with the XT2, that the signal strength indicator seems to be reporting a lower value than a similarly placed XT, whilst still operating correctly, which is definitely odd. I’m just pointing out that there could be an explanation for why they might differ.