Overtime rate calculation

Has anyone heard of an overtime rate being calculated in this manner?

Employee X worked 41 hours,

25 hours at $10/hr and 16 hours at $12/hour 

25 hours is 61% of 41 hours and 16 hours is 39% of 41 hours.

61% of 1 hour is .61*$10/hr*1.5=$12.11 and  39% of 1 hour is .39*$12/hr*1.5=$7.02.

OT rate = $19.13 ($12.11+$7.02) per calculations.  

If so, when/why is it done in this manner?

Thanks in advance

Nichelle