So if my assumptions are misconceptions,I want you to correct me please !.
Your assumptions are reasonably correct, but I'm not sure you understand some of the deeper implications.
The Pi not only run a Multi-Tasking Operating System, but what I'd call a "High Level" operation system (Linux!) It makes no pretenses about doing anything in "real time", and it's got LOTS of stuff that it's doing at the same time it is executing your program. The Arduino can pretty easily bit-bang a pin with a waveform accurate to sub-microsecond timing. A Pi ... not so much.
Of course, you CAN put a multi-tasking OS on an Arduino (especially the larger ARM-based boards), and you CAN program a Pi in bare-metal fashion (most of it, anyway. Some parts are not publicly documented.)
Another important difference is that Pi is all RAM-based; you load programs into RAM and then both the data and the code sit in RAM while it's running. An AVR Arduino has all the program code in Flash (and has very little RAM.)