I don't know why this is a surprise or hard for people to understand.
Khudobin put up good numbers with the Bruins, left for a good paycheck and put up bad numbers on teams with bad defense (CAR + ANA), then returned and struggled until the switch of coaches allowed for the Bruins to switch to a more modern defense, at which point Khudobin put up great numbers again. It's only been 2 games but he is back to struggling, this time with Dallas, who are not know for their defensive structure.
Halak put up great numbers for Montreal when they had a good team with good defense (a decade ago); with St Louis which always has good defense; and with Washington during his brief tour there. The outlier was his recent time with the Islanders who have bad team defense. Baring a run of injuries, there is no reason to think his solid play won't continue.
If the disparity continues between Halak and Rask into November, Halak may go down as the only goalie to steal the starting job from Price (09-10) and Rask (18-19).