To solve this expression, we need to follow the order of operations, which is parentheses, exponents, multiplication and division (from left to right), and addition and subtraction (from left to right).

First, let's simplify the expression inside the parentheses:

637 x 527 - 189 = 335,099 - 189 = 334,910

Next, let's simplify the expression outside the parentheses:

526 x 637 + 448 = 335,162 + 448 = 335,610

Finally, let's divide the two results:

334,910 ÷ 335,610 ≈ 0.9979 (rounded to four decimal places)

Therefore, (637 x 527 - 189) : (526x637+448) is approximately 0.9979.