There are 4 entries (wanted to make the playing field level for this
test). There are 2 with true for istf and 2 with false.
Then the difference here has to do with using orignum vs destnum as the join criteria. There must be more intersections for orignum than destnum, or your statistics are so far out of whack. It appears to be estimating 5M vs 500K for a result set, and naturally it chose a different plan.
