There is a simple reason: there are two different bobbin materials, butyrate and abs.
Butyrate was used in the old times and now only on Duncan's vintage style pickups like Ants, 59, PG, APH and CS Pickups. Butyrate is delicate, smells funny and it is softer.
Most others are made of ABS.
When new they have both the same color, over the years butyrate changes its colour to a yellower tint.