Check Hybrid System warning message

I own a 2010 Prius IV with 98,900 miles. Recently, I had the Check Hybrid System message come on and four yellow warning lights. A mechanic checked the code for me - the dreaded P0A94, usually indicating trouble with the HV converter/inverter. I got to the dealer who diagnosed the car and repaired it. The bill was to be $2200 - but he fixed it under warrantee for no charge.
The hybrid system warrantee is 8 years or 100,000 miles - whichever comes first. My car is 11 years old. Could anyone explain why the dealer honored the warrantee and fixed it for no charge.