Usually, the companies stating lower lm/w on 6500K triphosphors fluorescents and CFLs than the lower CCT ones (For example: Osram Duluxstar 23W 6500K, have 58 lm/w and the Duluxstar 23W 4000K and 2700K have 62 lm/w)
However, in CFLs in general, i have noted that 6500K CFL of particular brand and wattage, shines much brighter than the same wattage and brand CFL of 2700K while also lights much stronger (For example: Osram Duluxstar 13W/865, looks to me as lights stronger than the same lamp but of 827).
Why companies, states lower lumens for daylight color than warmer white colors.

The lumens are standardized for averaged eye sensitivity curve and (most important) are derived from the illumination level the given lamp provide within (given setup).
I think the way, how the eye respond to the cold vs warm light is given by the conditions humans were exposed to during the evolution history.
The colder lamps look brighter, because the colder light is more aggressive in a concentrated form (one, rather small bright spot).
Humans evolved for many millenniums with cold light color only in the form of diffused light from the sky, the concentrated one was either the direct sun (then on blue sky background and too bright to look at it directly) and/or rather low temperature fire.
Maybe as the sun (the only concentrated form of colder color light) is so powerful to actually harm the eye when directly looking at it, it build up an instinct to feel any focused cold light in that manner, so look out.
And as with the sunlight there was plenty of light, so eyes did not have to be as sensitive. But with the fire (the only artificial light available for humans for evolutionary significant time) there was only small amount of radiated power, so eyes had to be more sensitive.

Technically speaking, the efficiency in generating radiant power is about the same, regardless of the lamp CCT.

The "lumens" value that was used might have been the Visual Effective Lumens, which are different from actual lumens. However, I think that cool whites are brighter than warm whites.

The higher CCT sources do seem to appear brighter when viewed directly, but as for how bright an area appears when lit with X number of watts of a high CCT versus a lower CCT source, the lower CCT appears more effective, assuming photopic conditions.  I have seen this first hand when comparing 3 27W 6500K CFLs versus 3 25W 3500K CFLs.  The 3500K made the room appear much brighter.  Since the lumen is basicly a measure of light output corrected for the varying sensitivity of human vision with respect to wavelength instead of measuring actual luminous flux, the high CCT sources will naturally get a lower Lm/W number assuming all other variables remain constant, such as phosphor efficiency, which may not remain constant in the real world. Daylight phosphors may also be less efficient.