The researchers discount the possibility of life because on Earth the phosphorus found would be absorbed by life.
Phosphorus is concentrated by clays which might account for its concentrations:

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Phosphorus and clays.
Posted by Robert Clark on 1/5/2005 6:55:02 AM
In Reply to: UPDATE: Wishstone Rich in Phosphorus posted by jms on 1/3/2005 3:14:19 PM

Clay is known to be highly absorptive of phosphorus because of its small grain size.

Reducing phosphorus in aquatic systems using modified clays.
http://www.eidn.com.au/phos98.html

This page discusses the absorption of phosphorus on clays and other sedimentary materials:

13.3 Binding of phosphorus in sediments.
http://lepo.it.da.ut.ee/~olli/eutr/html/htmlBook_98.html

However, I've been informed that the APXS, Pancam, and mini-TES do not have sufficient resolution to see if the phosphorus is concentrated in the dark, smooth inclusions.
But perhaps the mini-TES could measure them if it were placed closer to the target rock than is usually done.

Bob Clark
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http://www.habitablezone.com/space/messages/363979.html

Clays have been found at the Gusev site and a recent paper claims the infrared spectra at Meridiani is best matched by a composition that includes 10% clay:

Mineralogy of the light-toned outcrop at Meridiani Planum as seen by
the Miniature Thermal Emission Spectrometer and implications for its
formation.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E12S03,
doi:10.1029/2005JE002672, 2006
"Abstract
Analysis of Miniature Thermal Emission Spectrometer (Mini-TES) data has
led to the recovery of a pure end-member spectral shape related to the
light-toned outcrop observed at Meridiani Planum. Data from the MER
Mössbauer spectrometer, APXS, and previous Mini-TES measurements were
used to constrain a spectral library used to determine the mineralogy
of the outcrop from this spectral shape. Linear deconvolution of the
outcrop spectral shape suggests that it is composed primarily of
Al-rich opaline silica, Mg-, Ca-, and Fe-bearing sulfates, plagioclase
feldspar, nontronite, and hematite. Conversion of modeled mineralogy to
chemistry shows good agreement with the chemical composition of the
outcrops determined by APXS. Details of the analysis procedure and
implications for the formation of the outcrop are discussed along with
terrestrial analogs of the ancient environment at Meridiani."
http://www.gps.caltech.edu/~tglotch/2005JE002672.pdf

It may be that life indeed was active in absorbing much of the phosphorus but the amounts seen by the rovers is because of the concentrating capacityof clays.

Bob Clark