Sales Quantity Variance

Question
A company makes and sells three products. Budgeted and actual results for the period just ended were as follows.
Product Budgeted sales Budgeted profit per unit Actual sales Actual profit per unit
Units $ Units $
X 800 10 700 8
Y 1,000 6 1,200 6
Z 600 12 350 16
—— ——–
2,400 2,250
What was the sales quantity variance?

Answer
The correct answer is: $1,325 (A)
Budgeted sales Std profit Budgeted profit
Units $ per unit $
X 800 10 8,000
Y 1,000 6 6,000
Z 600 12 7,200
———- ———-
2,400 21,200

Weighted average standard profit per unit = $21,200/2,400 = $8.8333
Quantity variance in units = 2,400 – 2,250 = 150 units (A)
Quantity variance in $ (standard profit) = 150 (A) × $8.8333 = $1,325 (A)

My question, why are they using weighted average?

Thank you