You know when a silicon cable starts getting stiff or a white plastic part gets yellow with time or when a coated plastic part starts getting sticky as you handle?

I suppose these would be examples of "chemical resistance".

It just means they are trying to use some more acceptable quality materials in it.

Some of their latest iems for example, are apparently flaking off their finish very early according to some owners around here.

Oil is a chemical. Presumably they resist skin oils better

Better glue I guess

Made them more chemically inert. Less likely to react with oxygen/oils/moisture/whatever was a problem before