In mushtarak, the shared element is the word-form; the different meanings need not themselves form one common universal.
“ʿAyn al-insān”: a human eye.
“ʿAyn al-māʾ”: a water spring.
The sound or written form ʿayn is shared, but the intended meaning is distinct. By contrast, “human” as applied to Zayd and Bakr does not become two unrelated meanings; one general concept applies to both.
The distinction matters later when an argument uses the same term in different senses and mistakenly treats them as identical.