Take the general case and look at the algebra. We have two numbers a and b.
We subtract 10 from each so we now have a-10 and b-10 as the numbers.
Let's add them together a-10+b-10=a+b-20. The sum of the original numbers is a+b. So there's a difference of 20 between the sum of the original numbers and their sum after each is reduced by 10.
Now subtract them: a-10-(b-10)=a-10-b+10=a-b+10-10=a-b+0=a-b. This is the same result as the difference between the original numbers a-b. Why did -(b-10) become -b+10 when the parentheses were removed? Well, -(b-10), which is the same as -1×(b-10), is expanded by using the distributive property of numbers. The minus sign (meaning -1) outside the parentheses is distributed (multiplied) by each of the terms inside the parentheses. Since b minus 10 is the same as b plus minus 10, the second term is minus (or negative) 10. When multiplied by negative 1, the product of two negatives is positive so -1×-10 is just 10, which means positive 10 or +10.
Understanding the distributive property explains what looked to you like an anomaly.
