Naturals Salon — Service charges @15%?

Address:

Chennai, Tamil Nadu, 600042

Went for hair cut. Total after purple member disc was 120. Aftr tax final amt was 138 (120+18 (15% of 120).

Here comes my doubt?
Service tax is 14.5 % currently. But they charge 15 % to all customers.

Could someone clarify me pls.
If they charge 0.5% extra to every customer. They probably would loot lot of money knowingly to their packet. Can someone look into the matter.