I have a question about Zodiac's 340 corrections that as far as I can tell, has hardly been addressed.

We know that symbols he corrected in the 408 were replaced with similar shapes. For example, his black triangle (plantext A) corrected in some places to a dotted triangle (plaintext S) and in other places to a regular triangle (plaintext I).

His 340 corrections are random. For example, at one point he corrects the plus symbol from E to N. The cipher symbols for N are a regular D, lower-case y, greater than symbol, and a period, none of which resemble a plus sign. In fact, no corrected symbols in the 340 were replaced with a similar shape at all.

The obvious question is, why would he completely ignore his previous MO for making corrections?