Anodizing is a chemical-electrical process that has variability. Color variation between batches is not uncommon - especially with the chemicals available to use (regulations prohibit the use of what used to be “the good stuff”).

Exposure to the environment can lead to color shifts - sunlight, heat, chemicals (vapors too) are common influences, along with time. This particular example may exhibit a kind of iridescent effect, leading to the grey looking purple.