<p>Look at percentile of score distribution in both tests. There is no need to make up a theory based on a few out-liner observations. Obviously one may do better in one test or the other. The percentile distribution is telling you what fraction of test takers are getting that score or higher. I am sure the percentile for SAT 1800 and ACT 34 do not match up at all.
For the adcom, they are not relying on this table as each school may put emphasis in different section scores. Some school mainly consider CR+M in SAT for instance. Some engineering school would look closer at the Math score. So there is no simple conversion chart that may be universal for all school anyway.</p>