Any ratings system is only as credible as its users. Unfortunately there appear to be plenty of users here that abuse the ratings system. You have people actually admitting that they rated a pin lower than they actually think it should be rated simply to bring the overall rating down/closer to what they think it should be.
In other words if a pin is rated an 8.7, and they think it should be rated a 7, they will rate it a 4 or 5 in an attempt to skew the results towards their opinion.