511 pulley worn in 5 runs

[fastinfastout]

I built the car as per manual, had about 5 runs with a 5.5 turn, and noticed the belt started skipping/slipping. The rear pulley has been worn down. The belt was not overly tight or loose, the belt is in very good condition. Seems unusual to get the pulley worn down so quickly. Slipper was not overly tight, just as per normal how I usually have my other cars. Is this the norm? The front looks new!