Sequence of widgets in dynamic sidebar

I’m relatively new to PHP, but I’m getting things sorted out and I’m pleased with how my blog’s looking (https://SheltieTales.com, if you care). But there’s one things that has me absolutely baffled — what are the rules for distributing widgets within sidebars?

I know that widgets that are accessible on the Presentation > Widgets screen (Sidebar Arrangement) are positioned into specified sidebars at specified locations. That’s not a problem.

But widgets can be defined in functions.php. For the life of me, I cannot figure out why widget A from functions.php gets put into my left sidebar while widget B from functions.php gets put into my right sidebar — and I certainly can’t find any pattern that explains why widget A goes near the bottom of my left sidebar while widget C, defined “later” in functions.php, goes near the top.

Clearly, I’m overlooking something, probably something obvious. Any pointers to information telling me how to suss this out will be greatly appreciated.

Jim