I've already mentioned it in the webmanga comments, but another common error is that digits may have been swapped. This kind of error also results in a difference that's a multiple of 9.
@conscript117: There are multiple variants, but the one shown in this chapter is a fairly standard abacus. The beads in the separated section each represent five of whatever unit their column is for: ie. in the ones column it literally represents 5, in the tens column it represents 50, in the hundreds column it represents 500, etc.