I did an experiment, I put a super-laggy servers for me up at the top of the list (us10.mcsg.in) and my usual servers which hardly lags following (au.mcsg.in servers). I get 2000ms (õ_õ) during this. I removed the us10.mcsg.in and it went back to around 100ms.
I believe I know why this happens - I also know that trusting your speed to the server in the Minecraft client's server list is a horrible idea, as that's only the speed you will get when connecting to that server at the same time as trying to make a connection to another server.
The problem with the server list is that it tries to connect to all the servers currently in your server list at the exact same time - as a result, it needs to divide up the work. Having two servers in your server list will make it take longer to connect to either one than just having the one would. Technically, putting a US server in the bottom of the list would also slightly effect your connection to the rest of them, just only a little bit instead of drastically.
Yes. When it divides up the work, it doesn't do it evenly - it gives priority to the servers in the top of your server list. However, all the other servers are still attempting to make a connection as well. If an AU person like you were to have all 28 US servers in the top of your server list, all following servers would likely get "can't reach server" from taking too long before they get enough priority to load up. Even if they did load up, they'd at least have a really, really low ping shown. (Assuming you actually scrolled down to them in your server list soon enough. It will not attempt to make a connection until you actually see it in your server list.) However, if I were to do the same thing (which I do already.) then they'd at least load up for me, however with a fairly low ping shown. As long as I can see if the server is "up" or not is all that matters for me though.
TL;DR: Put the servers you have a better connection to, such as servers in your region, at the top of your server list for the best performance. If you recently put a server in your server list or moved them around a bit lower, naturally you would get less ping to them. Don't worry about it unless you actually see the performance drop in game. (When in the server, the other servers will no longer effect performance.)
