This is the standard practice
The notification says that this is the minimum wages to be paid for each function in each industry. People working in the canteen are not working in engineering industry but in canteen industry. Also in engineering industry, the minimum wages are for skills required in engineering industry.
Skilled, Semi-Skilled and Un-Skilled are based on whether or not the person can work independently or needs supervision and constant direction.
It depends on the notification of the state government.
In karnataka, for textile industry, canteen staff is also listed and the minimum wages is given for it (just verified) at 106.8+50.22 so obviously we have to pay that. But if there is nothing stated, then you have to take the salary for canteen industry or function