Actually PUE as a metric for comparison favours longer established datacentres too, compared to potentially better designed more efficient ones that have just been built. So it's a bit of a "perverse" metric.
A room already filled up to capacity with IT equipment will score well on PUE because cooling, consumption etc will be balanced and close to it's design capacity, whereas a room that has just opened and has only begun to be filled up, will have power distribution and cooling equipment designed to deliver well in excess of what's being consumed, making it harder to get a good PUE score for year 1. Power and cooling equipment always has a design optimum and hardly ever scales linearly with what's being required of it. Compartimentalised (cold/warm aisle) data centre layouts make up for that a little bit, but make the data centre overall appear more expensive from a price per square foot / rackspace perspective.