From what I understand, there are two ways for an adapter to find and connect to a WiFi network: one is to scan for networks and join the first known network encountered; the other method is to scan for known networks, then check them for the highest throughput and connect to that network.
Apple uses the second approach to ensure the fastest connection. Other adapters can, and often do, use the first method. As you can imagine, the second method will take a bit longer to complete. That's why many adapter manufacturer's avoid using it, but using this second method always ensures that no matter what the WiFi configuration is, you get the best possible speed.
The first method will fail to give you the fastest connection under several circumstances, such as when the SSIDs are the same for both frequencies. Most adapters scan the 2.4GHz band first, so if they encounter a known network on this band, they connect and are done. They will simply ignore the 5GHz connection. To correct for this issue, separate SSIDs must be set up and the 5GHz connection must be given priority in the WiFi adapter's configuration.
As with most anything in life, there are always compromises.