A Better Way to Make Buttons in Sketch
Jon Moore

As somebody mentioned in another comment, I am learning so much with your articles. Looking at your file, I say the symbols for the text have the type left align. 
Is that to make sure you keep the exact left margin when changing the copy, and visually adjust the right margin?

For me it works better if I align center the text on its symbols, and stretch the button to fill the copy.