The 'best price guarantee' guarantees the customer will receive the lowest price for their ticket. Google will refund them if anyone gets a lower price.
The airline can release, for 30 seconds only, a much lower price. Then Google has to refund all their customers, and the airline might only sell a handful of tickets at the lower price.
Remember airline tickets aren't fungible. They have names attached, and can't be resold, so nobody can buy up thousands of tickets in a price dip and resell.
"Prices are measured between the purchase date and the departure date of your eligible itinerary." 
The key things to interpret there are "between" and "departure date". For example, I would interpret that as price changes on the day of the flight don't count. (i.e between is exclusive and date means the date, not date and time).
The airlines really are shooting themselves in the foot though... Google will end up with the biggest slice of the pie...