A kernel can be optimized for a certain processor architecture. For example, and Athlon kernel uses high performance features of the AMD Athlon chip, and doesn't work at an Intel Pentium 4 system.
If you compile a kernel, you have to choose what architecture you're building the kernel for.
I don't know about the source packages, perhaps they use common code that works at every PC platform?
But I'd expect to see i386 for binaries only