Callan certainly has an arrangement to ride for Michael Jarvis whenever appropriate (ie when Philip Robinson can’t and Callan is available). If the Ryan horse wasn’t a certain runner at decs stage, then it would make sense for Callan to be jocked up by Jarvis.
Callan, rather surprisingly, also seems to have picked up the ride on Royal and Regal, instead of, imo, the much better and better suited Robinson.
This has even been the case on cards when Robinson has rode other Jarvis horses, so in this particular case further explanantion is needed on top of Rory’s point.