It's ok if you do not understand it. Things can be true even if we don't understand them.
I was trying to be nice and point out why you were flamed, without being derogatory. So I'll frame it another way, we don't know what we don't know. Your post was one that came from either ignorance or taking negative shots. I think it was probably the former to start with, then drifted to the later. Ignorance being the lack of knowledge about something, ie how trade ins work. Being offered $18K trade in ( apparently over the phone) with a current year car that was just 3-4 grand more new is a very normal offer. Then they plan on only having 12-13K in the car after the sale, making up the difference on the new car. Then they can ask 18-19K for the car taking 17 ish, making 4-5K on the sale. The fact that you actually got $20K and paid under sticker on Mazda says two things, you made a great deal on your car (feel good about that), AND mazdas have a high markup on their cars. Subarus do not have a high markup. Atleast the regular models, the Sti prob has a little higher even tho it's loaded with more tech than the reg impreza. I "saw" a new Z-71 Tahoe stickered for around $50K bought for mid $30K. Obv the lower price the car is, the lower the mark up is, dollar wise anyway. Percentage wise they might still be fairly close.