running your stylesheet with saxon8 and 6 respectively produces

$ saxon8 fnum.xml fnum.xsl
Warning: at xsl:stylesheet on line 3 of file:/c:/tmp/fnum.xsl:
  Running an XSLT 1.0 stylesheet with an XSLT 2.0 processor
Error
  XTDE1310: format-number() picture is zero-length
Transformation failed: Run-time errors were reported

davidc@souldern /c/tmp
$ saxon fnum.xml fnum.xsl

    22,222.444444
    2.5555555
    999,999.444444
    22,222.444444

select="l.000" />

unicode 108 is an l so that's the same as

select="l.000" />

which isn't a legal format string, I think.

$ xsltproc problem.xsl problem.xml
XPath error : Invalid expression
#.000
^
compilation error: file problem.xsl line 5 element variable
xsl:variable : could not compile select expression '#.000'

What ouput did you want?

<xsl:variable name="numberFormat" select="'#.000'" />

produces

    22222.444
    2.556
    999999.444
    22222.444

in both systems.

<xsl:variable name="numberFormat" select="'#.000'" />
                                                             ^        ^
the inner single quotes save the day - many thanks :-)

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