Preferred wireless network

I was under the impression that whichever wireless network was listed at the top of the available networks list, would be used in preference to all others. But on my 2010 Macbook Pro running OSX 10.11.1 my preferred network doesn't get automatically chosen. If I select it, it stays selected but other than that it seems to select whichever might be the strongest signal. Whilst this may be a good thing in general, when trying to use Airplay with other wireless devices, it causes all sorts of dropouts.
Anyone else notice this behaviour, or have any recommendations?

Not at all. When you set up Networking, did you opt for "Automatic" as location and this still displays in System Preferences > Network?

Sorry for the late reply.
Yes. I opted for Automatic and it still displays in System Prefs.
I have to admit, I have read the explanation of this setting on Apple's web site, and I still don't understand what they are talking about.
(Probably having a thick day today)

Sort of an aside to this query.. When I look in the Network pane within System preferences, and select the Advanced tab, I can see other networks this computer has joined. But here's the weird thing, it is showing networks I have accessed BUT not with this computer. It is showing networks I accessed whilst on holiday this year in Italy. But I didn't take this computer with me. I did however, take my iPhone to access these networks. Is this something to do with iCloud?

Highlight these unwanted networks and click the little (-) button to move 'em out.