Spreading phlox is common within its range, though typically reliable resources suggest different ranges. USDA GRIN has a more restricted western range for Phlox diffusa than USDA PLANTS database; the latter suggests Phlox diffusa ranges across much of western North America, going as far east as South Dakota and into New Mexico. This suggests some taxonomic confusion, and this is borne out by some floras recognizing multiple subspecies and varieties within Phlox diffusa, and others using the term “highly variable”.
According to some references, today’s plants could also be recognized as Phlox diffusa subsp. longistylis, differing from Phlox diffusa subsp. diffusa in having larger corolla lobes (8-10mm long instead of under 8mm in one reference, 5-9mm in another) that are nearly as wide as long (instead of twice as long as wide) and styles 5-6mm long (instead of 2-4mm). However, one reference also notes that some plants do not cleanly fit into either subspecies (i.e., through a mixture of characters such as short styles but large corolla lobes), explaining that this is why some botanists choose to recognize only a single variable species. This would be my inclination in this case.
Flower colour is another variable characteristic. In my experience, pink to lavender is far more common, but Phlox diffusa is one of the easiest species to find white-flowered individuals. While the relative proportion of white to pink individuals is certainly a factor, the task is also made easier by the showiness of these mat-growing plants and the general lack of “colour competition” where they grow. Simply, few other flowering plants inhabit the same niche (mesic to dry rocky slopes and rock outcrops), so the spreading phlox tends to stand out. White-flowered variants is another BPotD series I could do, I suppose, as I suspect I have about 20 species photographed with the typical coloured flower and the white variant. Or, perhaps another half-decade of flower photography will yield a presentation on the topic.
Additional photographs of this species, including the range of flower colour variation, are available from the Burke Museum: Phlox diffusa.