變動成本法 與 歸納成本法 - 會計

題目:
B Corporation has the following standard costs associated with
the manufacture and sale of one its products:

直接材料 3.00 per unit
直接人工 2.50 per unit
變動製造費用 1.80 per unit
固定製造費用 4.00 per unit
(based on estimate 50000 units per year)
變動管銷費用 0.25 per unit
Fixed SG&A expense 75000 per unit
During 20X1,B manufactured 51000 units and sold 48000.The selling
price per unit was 25元. All costs were equal to standard.

Based on variable costimg,the income before tax for the year was?

我的想法是...
Sales 25*48000
- VC (3+2.5+1.8+0.25)*48000
-------------------------------
CM 17.45*48000
- FC (75000+4*50000)
-------------------------------
OI 562600 ANS:562600

可是他的答案卻寫558600 也就是我的答案再減掉4000

唯一的解釋就是減去"生產數量差" (50000-51000)*4=4000(有利)

...可是...減掉它幹麻 = = 而且變動成本法不是應該沒有生產數量差嗎??

請板上的高手為小弟解惑...小弟駑鈍 = =

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