I meant “poll” in the general sense of the word. As in: the rate at which the mouse takes readings on the surface in front of it. I didn’t mean to conflate this with the USB clock speed (which also happens to be referred to as the “polling rate”) Perhaps I should have chosen a different word. Let’s just call it the “update rate”.
The way it works is like this: Every time the mouse detects that it’s moved a “dot” (an arbitrary unit of physical resolution on your desk which has nothing to do with pixels), it registers an update in that direction. In this way, the update rate (or “polling rate” as I was previously calling it) is directly controlled by the DPI setting. A lower DPI means that the dots are farther apart and the updates occur less frequently.
As the updates are recorded by the mouse drivers they track two separate numbers for +/- x and +/- y. Every time the USB port is read these totals are sent to whatever software is interacting with the mouse driver. This happens once every 8ms at 125hz (the default) or once every millisecond at 1000hz (the standard for most gaming mice). The software (IE: Borderlands 3) which receives these updates can do whatever it wants with those +/- totals, but it typically just multiplies them by a sensitivity number and translates them into in-game viewing angle adjustments.
This process ends up making DPI feel like it’s only adjusting the sensitivity, because more frequent updates end up causing the +/- totals to have higher net values. The game then ends up translating these into larger angle adjustments per USB cycle than you’d get with a lower DPI.
If you were to lower your DPI to something horrible like 10 you’d absolutely start to notice the lack of precision. It’s arguable that anything above 1000 DPI or so is unnoticeable and does effectively translate to a sensitivity adjustment, but I guarantee that at a technical level it’s also more physically precise.