I don't see any reason to be suspicious of the coin.
There are many minor variations in calligraphy.
There is also a wide variation in the quality of manufacture. Some are very sharp and well defined while others are soft or muddy.
Depending on the economy at the time, they might be made to a different size and weight, or with a different alloy.
I agree with your interpretation of Kang Hsi and Boo Yuwan, and would personally just leave it at that.