I think it's the light penetration from the HPS. The CFL's lumens drop off sharply when the light meter is taken away from the CFL.

This is why it is advised to keep CFL's very close to the plants. The HPS is hung much higher and retains the lumens over greater distances.

In my first grow in my signature I used a cheap light meter to measure the output from my LED light.

Hope this answers your question.

Take Care

S.L.