Also worth looking at the historical data in http://www.eia.gov/totalenergy/data/monthly/pdf/sec1_4.pdf http://www.eia.gov/totalenergy/data/monthly/pdf/sec1_4.p

Also worth looking at the historical data in http://www.eia.gov/totalenergy/data/monthly/pdf/sec1_4.pdf http://www.eia.gov/totalenergy/data/monthly/pdf/sec1_4.pdf. That shows that this was also the case briefly in the late 90s, and consistently before about 1990.