I have been thinking it has a lot to do with canopy temps. HPS emits an 825nm spike of infrared radiant heat that may raise the temp in the buds, causing a fluffier bud and extended flowering (smix of mature and immature calyxs). Higher temps should also result in a higher yield on the scale if CO2 level, water and nutrient availability was sufficient.
Ace of Spades run with warmer canopy temp (~82-85f with bare 600W HPS) Great bud but fluffy and slow/sloppy finish.
Same Ace cutting under HPS with a cooler winter finish. The buds that were shaded have noticeably more color.
Same Ace cutting run under LED with canopy temps (75-78f) Fast clean finish, more ice, more flavor, increased density. One of the most popular smokes I have.
A friend (HPS grower) gave me a selected cutting and said it was an 11 week finish but good yield. Normally I would not run something like that but I suspected I could get it to finish quicker and wanted to give it a shot. Glad I did, finished in 58 days under the COBs with a nice clean finish and amazing flavor (looking for a pic). Just ran that same cutting under COBs and tried to bump the canopy temp up to about 80f (cool summer finish). Here she is at 57 days, probably pull her out of the flower tent in a few more days and then sit a few days in the dark.