under/over absorbed overhead

The budgeted overhead absorption rate for variable prduction overheads in department A of Lublin’s factory is $3.00 per direct labour hours and for fixed overhead is $4.50 per direct labour hour. Actual direct labour hours worked exceeded the budget by 500 hours.
If the expenditure were as expected for variable and fixed overheads, the total over-absorbed overhead for the period would be:
A $507.50
B $1,500.00
C $2,250.00
D $3750.00
Hi. Could u pls explain for me why when the expenditure are as budgeted but actual and budgeted production activity levels are different, only fixed overhead can be under or over absorbed ?
thank you so much !