Political capital

If it was a favor and he asked you to remind him, he wanted you to owe him more, i.e., he wanted to maximize his political capital in you.

By being terse, you can be seen as minimizing (his maximizing.) So, he is minimizing in return.

Not saying it must be the case, just saying it could be. Of course, you know the context to better judge.

 

Political capital is a tricky thing. Bosses like to be taken seriously, especially the insecure ones. If you felt the need to amend your initial message, it's a good indication you need one.