That doesn't explain why it's smaller than zip though.
PNG filters might also play a role in here, because the difference to the previous pixel row may be easier to encode than the ASCII text on its own (there are fewer and smaller delta values to encode).
Isn’t the loss of precision what is “compressing” the file? The Shakespeare corpus probably doesn’t use any Unicode characters.
An encoding (like ASCII or UTF-8) describes how bytes are mapped to such symbols. ASCII only describes 128 characters whereas UTF-8 can represent any Unicode symbol in bytes.
Notably though, UTF-8 is a strict superset of ASCII. That is, every character ASCII represents is represented by the same bytes in UTF-8. So encoding Romeo and Juliet in ASCII or UTF-8 results in the same exact file.
I think the small size difference is just the difference in overhead of the framing formats (Zip encodes eg. local file headers and the central directory record) and compression options.