Does anyone know why a Workhorse7 could not run 4x24w t5ho?

[Delphinus]

I was looking at Ocean Aquatics info page on Workhorse ballasts and noticed the following:

Description
Lamp # F24 HO
Workhorse 3 - 1-2
Workhorse 5 - 2-4
Workhorse 7 - n/a

(From http://oceanaquatics.com/store/produ...-5-T5-Ballast/ )

Could anyone speculate why a WH7 couldn't drive 4 24w t5's when a WH5 could? I happen to have an unused WH7 and was hoping to use it for this purpose, so I'm a little disappointed to find this nugget of information .. I'm hoping it's just an oversight and that I could just run the ballast to run the lights.