I fear when it comes to calculations with percentage the main problem are users that don't know how to enter the formula right.
My math teacher always said: NEVER use the percent and the root keys of your calculator because they will only cause trouble. Do your calculations the "real" way (x/100*y) instead of percent and (x^(1/y)) instead of root.
It seems he was right
I don't see what's wrong with that. The way the computer interprets your command is 100 minus 15 percent of 1 which is 100-0.15=99.85. I assume you want to subtract 15% of 100 to get 100-15=85? Basically the % operation just divides the value by 100. So 15/100=0.15. You still have to multiply that by something (in your case 100) to get back to the percentage of the value you want.
Its not an error with the calculator its an error with user input. Trust me I see this day in and day out because I am a maths teacher.
A calculator will only do the calculation you tell it to do, you still have to know some maths in order to input it correctly