It's my book, my memory.
At the NYABF 2019, Karel Martens in conversation with Prem Krishnamurthy, re: facsimile reprint of Karel Martens Re-Printed Matter
When asked the difference between other books Karel has designed vs. Printed Matter, he replies that the latter is his "memory," a helpful archive that he references due to his bad memory, and his diary.
Reference link: https://www.romapublications.org/Roma151-300.html