Clarification on the SEA-TAC VFR map
Your question is flawed because you have labeled the airspace incorrectly:

The thin circular magenta line you have labeled class C is actually the edge of the 30 mile mode C veil around SEA.

The shaded magenta line you have labeled class D actually depicts the area where the class E floor is 700’ AGL.


If you want to see how class C airspace is normally depicted, look North to NAS Whidbey Island.
If you want to see how class D airspace is depicted, come halfway back to Paine Field in Everett.  (There is also class D around SEA, BFI and RNT, but they overlap each other and the chart is pretty busy in this area, so Everett is a cleaner example.)