Microsoft Knowledge Base Article 230623 contains the following summary:
A standard Windows configuration functions as a single-route software
This operation is in contrast to a multiple-route hardware
router that may also exist in some environments.
Sometimes administrators attempt to configure Windows to perform
like a multiple-route hardware
but because Windows is not designed to do this,
these attempts do not succeed.
One key difference between Windows routers and hardware routers is that a standard Windows configuration maintains a single route table for all kernel-level routing decisions. This means that when a WinSock call is made, a single route table is consulted to determine the best route for all network traffic.
A hardware router, or multiple-route router, can usually be configured to maintain a route table for each interface. The ability to provide this type of functionality in a hardware router usually requires additional microprocessors to accommodate the added complexity of the routing algorithms. With this added processing power, hardware routers are able to maintain more than one routing table for routing decisions, and are therefore considered multiple-route routers.
A standard Windows configuration is a single-route router by design. This means that programmers may not be able to perform certain activities of which a multiple-route router is capable.