In word, why do the same underscores appear thick and thin?

Because underline can be modified and adjusted, here is a simple demonstration for everyone.

Computer: MacBook Air

System: macOS 12

Software: WPS20 19

1. First, when we open the word document, we can see that the thickness of the underline is different.

2. Then we select the content to adjust the underline thickness, and click the underline button in the upper left corner.

3. Then the list pops up, and you can see underlined lines with different thicknesses. Click Select.

4. After the setting is completed, you can see that the underline has become the same, as shown in the figure.

I hope it helps you. If you have any questions, please leave a message. Thank you.