question from bpp

George manufactures a product which uses two types of material, A and B. Each unit of production currently sells for $10. A local trader has expressed an interest in buying 5,000 units but is only prepared to pay $9 per unit.

Current costs and revenues are as follows.
$’000 $’000
Sales 350
Less production costs
Material A – 1 kg per unit 25
Material B – 1 litre per unit 50
Labour – 1 hour per unit 75
Variable overhead 50
Fixed overhead 25
Non-production costs 25
Total cost 250
Budgeted profit 100

The following additional information has also been made available.
(a) There is minimal inventory of material available and prices for new material are expected to be 5%
higher for Material A and 3% higher for Material B.

(b) George has been having problems with his workforce and is short of labour hours. He currently has
the capacity to produce 36,000 units but would have to employ contract labour at $3.50 per hour
to make any additional units.

(c) Included in the fixed production overhead is the salary of the production manager. He is stressed and has threatened to leave unless he receives a pay rise of $5,000. George would not be able to fulfill any new orders without him.

Required
Evaluate whether George should accept the new order.

Solution:

Workings
Current production = 350,000/10 = 35,000 units

Current cost per unit of Material A = $25,000/35,000 = $0.71

Current cost per unit of Material B = $50,000/35,000 = $1.43

Current cost of labor = $75,000/35,000 = $2.14 $

Incremental revenue (5,000 x $9) 45,000

Incremental costs
Material A (1.05 x $0.71 x 5,000) (3,728)
Material B (1.03 x $1.43 x 5,000) (7,365)
Labour [(1,000 x $2.14) + (4,000 x $3.50)] (16,140)
Fixed overhead (5,000)
———-
Incremental profit 12,767

i got the solution…..but my question is For those 5000 units why variable overhead cost per unit aren’t considered and taken in cost