Because they have 10 BTC and need 8 BTC for spa-massage and makeup. Should they give just 1.9 BTC not 2 BTC for dinner? Ok. Wait! To pay for the massage and makeup... it's necessary to have 8.1 BTC.. But what if massage and makeup r payed separately, then it's 8.2 BTC... Hey, I need a computer!
You're mixing stuff.
10 BTC is everything she has to spend during the day? An amount that barely pays spa + son's dinner? I doubt. She probably has more, and the amount is probably is not "round", as probably is your bank account balance right now (or do you have an exact power of 10 as money in your bank right now?)
You're thinking on "$10 bills" or other round values like that - which may still be an option, you may print BTC bills.
But if the transaction is electronic, she'll probably have a "not round amount" of coins, will transfer 2 BTC to her son, and keep having a "not round amount" of coins. Where's the problem?