This is because 100% + 10% = 110% = 1.10
Example:
An item costs $37 before tax. Sales tax is quite high at 10%. The tax alone is 0.10*37 = 3.70 dollars and the total price after tax is 37+3.70 = 40.70 dollars.
Or as a shortcut, 1.10*37 = 40.70
