There's a bit of latency involved in establishing a connection to a network. If I had to hazard a guess, what's happening is something like this:
The MacBook sends out several requests for network connections simultaneously.
Some of them come back right away. Some take longer to process, depending on bandwidth, network latency, packet loss, whether or not the network is using encryption, mac address filtering, and whatnot.
It's quite possible that the server with the stronger signal is SLOWER, and takes longer to respond. But, because of some caching system (either at the server or in the MacBook, or both), when you turn your WiFi off and back on, you get that stronger connection.
This is, of course, all an educated guess.