This seems the most likely explanation -- compounded by the fact that us kids reading the books were just beginning to learn how to read. After reading the art

This seems the most likely explanation -- compounded by the fact that us kids reading the books were just beginning to learn how to read. After reading the article, I do vaguely recall some childhood cognitive dissonance over the fact that it was pronounced "steen" but spelled "stain"... but my main memory is of the pronunciation, not the spelling. If you'd asked me before reading this, I'd've sworn up and down it was spelled "Berenstein".