The numbers are absolute values. 1250 is the median ("average" or no description) for Dwarves in the base game, 1000 the median for Humans.
The descriptions are relative values, as defined by the
MENT_ATT_RANGE tag of the creature in question - or of its specific caste, if they differ from each other. Base game Dwarves don't differ, but Masterwork (1.9.5) in particular has several castes where the strength attribute is distributed differently.
For comparison, this is the default
CREATURE:DWARF entry for strength:
These are from Masterwork 1.9.5 (abbrievated)
[PHYS_ATT_RANGE:200:700:900:1000:1100:1300:2000] -- default, not in the raws
As to your specific example, almost every Dwarf in your list is from one of the fighter castes. For them, an absolute strength value of 3750 or less is already "weak", and 3500 or less even "very weak". The one "mighty" Dwarf with just 1995 absolute strength value is from one of the non-fighter, non-noble castes.