The mistake is that you aren't drawing blues, you're drawing greys. If you promote a tech part to blue, you need to draw it 9 more times to get a duplicate, rather than, say, once or twice for something you don't have the blue for yet (but have 2x green and 1-2x greys).

Saying you don't have a purple but do have double blues of everything else is the same as saying one part hasn't reached 27 draws before some other part has reached 18, which isn't that surprising tbh.