This seems totally bizarre (a person is, after all, responsible for their own taxes) until you put it together with something else he's told us: that in his 1996 deposition he misled the court about his dealings with Schreiber because he had no responsibility to volunteer information and he wasn't asked the right questions. In his mind, when he said he had had no dealings with Schreiber he meant re Airbus, and when he said his only dealings with Schreiber were to have coffee he had no responsibility to add and collect envelopes full of thousand-dollar bills.
Now it seems that he is "not volunteering" something again. A possible explanation is that he doesn't want to say under oath that he took no more than $225,000. In his mind, he may think it's perfectly legal for him to admit to taking $225,000, even if he really took a lot more - but he doesn't want to say he only took $225K.
How much more? I'd guess $5M. Way back when, in the investigation that led to the defamation suit, the RCMP thought Schreiber paid Mulroney $5M for securing the Airbus purchase. In that suit Mulroney testified that he had had no dealings with Schreiber. Then we learned that he'd got $225K from Schreiber (and, as it happens, forensic accountants believe that that money came from Airbus). Maybe the entire original allegation was true. Who knows. It doesn't seem like Oliphant is going to get to the bottom of it.
...Or it could be something else entirely. Mulroney has played this whole inquiry fast and loose. He is obviously in damage-minimization mode. He may lose his law license and have to pay some back taxes, but doesn't seem to be jeopardizing anything else, even though his flimsy excuses for his behavior don't pass the smell test. He has tried to obfuscate as much as possible (the phony "fourth article" in the Globe series by Kaplan; the near-tears he said were caused by reporters laughing at him (refuted by witnesses); the repeated self-righteous rants about conspirators. He seems to be playing to his supporters - those loyal enough to cling to anything in the hope he's innocent.