Lucounu-
That is true to a certain extent- the main difference is the starting point. However, the 2-5 test has a discontinuation criteria at the end of Algebra I topics and the question bank does not contain all the topics that are on the 6+ test.
I know you know this, but how do you know? I believe it to be entirely possible to get a 273 score by taking the 2-5 test. DS's score placed him at the 92nd percentile for 11th graders as well as 92nd percentile for those who had finished Algebra 1. He also told me that the questions got past his knowledge level and he had the feeling he had missed some at the end, same as the other times he took the test.
I know because I've actually sat the test, looked at the question bank, sat the 6+ test and looked at the question bank. I'm not saying your son didn't get 92nd percentile for 11th grade, just that the score may not necessarily be the same on the 6+ test because the question bank is deeper and goes through a higher level of math.
For example, the 6+ test includes (in just the Algebra subtest) parabolas, linear and non-linear equations, estimation of infinite sequences like 1/n, compound interest, quadratic functions (not equations but functions) and some log equations. These questions are not in the question bank for the 2-5 test.
Overall, NWEA MAP (like IQ tests) are not good at sorting out the higher ranges of the test (generally 270+) because there simply aren't enough questions, nor enough advanced questions for it to discriminate. Many schools are now stopping MAP in 10th grade because they realize the top end isn't deep enough to be useful.
You might find this interesting- this is from an NWEA training report-
http://moodle.bismarckschools.org/mod/book/view.php?id=15583&chapterid=1510
