CSS Test for swimming

Question for those of you that have used the CSS calculator on Swimsmooth.com for swim training. Can someone enlighten me as to how someone who swims a slower test for the 400 and 200 can have a faster CSS number than someone who swims the 400 and 200 faster? For example: input the following times into the calculator from the swimsmooth website below and you get a faster CSS for the slow test than you do for the faster test:

http://www.swimsmooth.com/training.html

slow test times: 400=6:19, 200=3:14
faster times: 400=6:15, 200=2:55

I’m not questioning it, I’m just wondering why.

Great photo on that site with the gal in the red cap over rotating to breathe.

Use the tables on the 2nd link

http://www.google.ca/search?hl=en&client=ms-android-samsung&source=android-browser-type&v=133247963&sa=X&ei=-NwpT5X-CvDZiAKlzpDTCg&ved=0CB0QvwUoAA&q=jon+urbanchek&spell=1&biw=240&bih=339

Your answer lies in the definition of CSS and the faster speed fall-off rate of the faster swimmer in your example.

Maybe this post will answer the question:
http://swimsmoothperth.blogspot.com.au/2012/01/common-query-about-css-results.html
Response to the same question from the Swimsmooth squad, we just did the 200/400 CSS test about a month ago.