This is rare but the only reason this could happen again is if the only buses ready for pull out of the garage is an XD60 again. You can’t just pull out any bus you want on any line, they have to be checked out and assigned if ready.

Yep that’s exactly it. It’s whatever they have available to pull out, which is also the same reason why you’ll see suburbans tossed on the most random Hilton lines.