That's a bug in the roll20 sheets. I've noticed it too. We don't really use the in-game character sheets at all (See basic rules post, 3rd paragraph). If you would like to make something you can refer to in your macros, I suggest making Attributes in the Abilities and Attributes tab for each modifier (STR, WIS, CON, etc.) and then what you can do in your attack macros is say something like [[1d20+ @{selected|STR} + (other mods, like proficiency, expertise bonus, etc)]] vs AC. or whatever is appropriate. You can additionally make attributes for things like half level, proficiency, and expertise bonuses (which, remember only get added to attack rolls, not damage rolls) if you are feeling super detail oriented and like the idea that you can just change one number as you level up and it will adjust all of your macros.
