Tone and Register

There is this aspect of composition writing that is corrected based on 'tone and register' i.e. is your tone too informal for a formal letter? Speaking to your uncle like he's a teen? etc..

It's amazing how people can teach these things and be totally oblivious to actual tone and register in real life.

My supervisee is highlighted for being late to work too often. I am tasked to address the issue.
I sms her 'I need to speak with you. Is it convenient to call you now?'
Reply 'No time. Let's talk tomorrow.' (person that I have to rank come performance bonus time)
I sms ' It'll only take 5 minutes. It's quite important.'
Reply 'Can sms? Not convenient to talk.' (person whose report I have to write)
I sms 'Your name was highlighted at the managment meeting. It was noted that you've been late to work frequently. Do you face any problems in coming to work on time?'
Reply 'No! I've only been late once! And I do my best to walk fast to work. Mdm XYZ can prove that. Anyway, I'm not the only one who's late to work!' (person who is supposed to be a mentor and role model. )

Wah LAO eh........What happened to tone and register?