XM par 10K 20K 15K Huh?

[Covey]

I am in the process of setting up lighting for my new tank and I was looking at XM bulbs for my halides. The tank is 29" deep and I was going to go with 400W setups. Here where the question is why is the PPFD so low on the 15K XMs?

Personally I don't like how yellow the 10Ks are and I was worried the PAR on the 20K wouldn't be high enough. Well I was set to order 3x400 XM 15K went someone referred me to your site.

http://www.reeflightinginfo.arvixe.com/index.htm

By the way thank you for setting that up. Real numbers not conjecture. A rarety in this hobby.

Anyway the PAR on the 15k XM is horrible.
According to your numbers on a m59
40% lower than the 10K XMs
19% less than even the 20K XMs

It seems to be generally excepted than as you go bluer the PAR goes down I was wondering what was so unusally to account for this bulb being out of the norm.