SAT scores have finer granularity than ACT scores. Each ACT score is concordant with three or four SAT scores. For example, a 36 ACT is concordant with 1570, 1580, 1590, 1600 SAT. So if a student does equally well on the ACT and SAT, and the student is in the 36 ACT range, then the student needs to be in the top end of that range to get a 1600 SAT.
In addition, the ACT composite is an average of section scores, while the SAT total is the sum. This means that the ACT test taker need not get the top score on each section (e.g. 36,36,35,35 → 36 composite) to get a 36 composite, while an SAT test taker needs top scores in each section (800,800 → 1600) to get a 1600 total.
But unless the student is trying for something like University of Alabama’s Presidential Elite scholarship (4.0 GPA with either 36 ACT or 1600 SAT), these differences at the top of the range are unlikely to matter.