Sales qty variance (kaplan exam kit n.120)

Hello John,

Another doubt with Kaplan’s offered solution. Here is the problem:

You have been provided with the following information relating to the three products:
Product X Product Y Product Z
Demand (units) 1,000 2,000 3,000
Selling price $15 $20 $30
Profit per unit $2 $5 $2

Actual sales for the year showed following results:
Product X Product Y Product Z
Units sold 1,100 2,050 2,800
Sales value $17,050 $38,950 $86,800
Profit $3,080 $10,455 $6,160

What is the sales quantity variance?

What I have done is a simple application of the rule:
?ctual quantity at standard contribution=1,100*$2+2,050*$5+2800*$2=$18,050
Budgeted quantity at standard contribution=1,000*$2+2,000*$5+3000*$2=$18,000
Therefore sales quantity variance is $50 adverse.

However, Kaplan offers a different solution including sales mix (?), copied literally below:
AQ AM SP AQ SM SP BQ SM SP
units $ units $ units $
X 1,100 2,200
Y 2,050 10,250 *
Z 2,800 5,600 @$3 @$3
———————————————————————————-
5,950 $18,050 5,950 $17,850 6,000 $18,000
———————————————————————————-
*=$3

Sales quantity variance = $18,000 – $17,850 = $150

Why are they including mix variance into the solution, since it it irrelevant and if SM means standard mix, how come our results differ?

Many thanks i advance
Regards
Marko