Why am I getting notice to upgrade two different software systems on my PacBook Pro

Unfortunately Apple's nomenclature may not be precisely clear when looking at System Settings dialogs.

An update is a point revision of the currently installed OS, e.g Sequoia 15.6 to Sequoia 15.7.

Updates tend to provide improvements and fixes for the little problems that show up within the supported life of an OS.

An upgrade is moving the OS from one major release to another, i.e. Sequoia to Tahoe.

Upgrades, well they often break things, albeit usually temporarily, until they start to get their own point updates. Veteran Mac users will often wait a bit and watch the forum boards for sign of common issues before applying upgrades to their computers.

You may very well have both an update and an upgrade available to you on your Mac if you are currently running macOS Sequoia. Which you choose is up to you.

You don't have 2 operating systems, but Apple offers the choice to update to 15.7.1 if you are still on a previous version of macOS 15 Sequoia if you don't need or want to update to macOS 26 Tahoe yet.

Since Tahoe is a major update, Apple allows users to remain on Sequoia while getting the needed security patches, so as to not upset any workflows that may not have yet been completed validated to work on Tahoe.

At the end of the day it's your choice whether you stay on Sequoia, or upgrade to macOS Tahoe.