Hello,
Please remember that mirrorlist picks a number of mirrors from a give country at random .. and goes to 10 ... if there are are more than 10 mirrors in a given country, then a given mirror may not show up.
In our country is only 3 mirrors at the moment (one them is on 100mbit, now a lot of home users have more...)
Each repo gets a random number of mirrors and the mirrorlists are updated at intervals throught the day (every couple of hours) .. so updates has different mirrors than os and extras, etc. The actual list of 10 mirrors is random. Once the list of 10 is obtained, then yum uses fastestmirror to rate those ten mirrors, BUT there is no way to ensure a specific mirror makes it into the initial random 10.
Ping not allways finds fastest mirror, but adding prefer to /etc/yum/pluginconf.d/fastestmirror.conf helps. Today updated ~hundred servers, all of them used correct mirror, but updates was allways not from our country. Yes now all things goes like it must be, so lets say it was temporary... Thanks for attention. Martynas