The authors conclusion for that test was:
"The most important conclusion in my opinion is that the efficiency of converting electrical energy into PAR light energy is not that different for the several bulbs and technologies included in the sample. The majority of bulbs in the sample deliver approximately (within a 20% range) the same amount of uE/s/Watt in the 400-700 nm range, about 1 uE/s/Watt. High intensity discharge lamps and high-end fluorescents tend to be more efficient, but not by a large factor. "
You pointed out an anomaly, not the general trend. The author seems to think that the differences even across the various technologies are minimal. The differences across bulbs rated the exact same is yet smaller.