No wonder...I tried to multi-city SFO/SEA/ICN (outbound), then ICN-SEA-SJC (return) on delta.com about two weeks ago, and it priced out about 8x as much as the SFO/SEA/ICN/SEA/ICN (in full Y, no less).
Yet somehow, Kayak was able to piece it together and send me to delta.com to book it at a V fare.
If you read the article text, two things stand out:
*Delta has the highest multiple on the examples stated, charging 10x versus up to 2x for AA or United
*Separate one tickets means change fees for each reservation
Bad news.