Your reading that wrong. Google has 54,000 people working for them and the meadean person working there right now has been with the company just over a year. However, compared to say IBM nobody has been with the company 20+ years and they only had 30,000 people in 2012 so the average is vary influenced by new highers and few long term people to balance it out. Worse there data is stale as people don't keep updating how long they have been with the company.

PS: The real number you want when comparing working conditions is turnover and even that get's influenced by rapid growth.