The guards were never able to distinguish new marks from old ones, but a few of the guards had a sense of what the volume was before they’d gotten their forty-eight hours off, or gone on vacation, and they recognized, if nothing else, when the randomness expanded. Now, I understand why they would have seen this as a pattern, and perhaps there was a pattern there after all, for I confess myself that had I been confined for another year or two the walls would have been full, there would have been no marks at all, just a wash, a new patina whitening the walls with marks of memories, all running together as if the memories themselves aspired to be the walls in which I was imprisoned, and that seemed just to me, that would have been a worthy pattern to have made. But it was not to be. Everything disrupts. The guards seemed to understand that my marks had meaning, so surely they can be forgiven if their error was one of interpretation.
How would you explain the last sentence: if their error was one of interpretation?