Counting with a kid

Today, while going through counting with Rohail using a calculator:
S2: (types in 14) whats this?
Rohail: fourteen!!!
S2: (enters 15) what this?
Rohail: fiveteen!!
S2: No its fifteen.
Rohail: no fiveteen!
After few mins fifteen it is!
Me: (enters 16), whats this?
Rohail: sixteen?
S2: yes!
Rohail: whyyyy ????? 
=p