Find pivot index question

i want to ask in this question which was ask by monu sir to complete it from home
here’s the code which a got from google ;-
class Solution {
public int pivotIndex(int[] nums) {
int totalSum = 0;
int leftSum = 0;

    // Step 1: Calculate the total sum of the array
    for (int num : nums) {
        totalSum += num;
    }
    
    // Step 2: Iterate through the array to find the pivot
    for (int i = 0; i < nums.length; i++) {
        // Right sum is totalSum minus leftSum minus the current element
        if (leftSum == totalSum - leftSum - nums[i]) {
            return i; // Found the leftmost pivot index
        }
        // Update leftSum for the next iteration
        leftSum += nums[i];
    }
    
    // No pivot index found
    return -1;
}

}

why in step two we r calculating the right sum immediately , don’t we first need to calculate the left sum then minus this it from the total sum and what if we first calculate the prefix some then do the other task ?