Homebrew builds individual binary bottles per package and per macOS version, but not per target architecture.
In other words: at bottle creation time (server-side), Homebrew tells the compiler to output code that runs on older CPUs. The downside of this is that end users who install bottles never get to enjoy the benefits of latest CPU instruction sets, such as SSE 4.1/4.2.
From time to time, Homebrew raises the bar to keep the balance between performance and compatibility. From Homebrew 1.9.0, bottles are built to use the features of the Penryn/Core 2 microarchitecture or higher, or in other words: Homebrew no longer supports CPU microarchitectures older than Penryn/Core 2.