I will say that it may be due to the mumble server being located in the Nord-Pas-de-Calais region of France. (pretty place for sure.)
Since Tf2center is a global service, usually you are used to playing on regional game servers. Currently we only have one mumble server. Connections between country "x" and France may be an issue.
On a side note, a couple people have raised a really good point that the ping between France and, say, Australia can at some times affect game play. Discussions have taken place about regional mumble servers. This hasn't really been scheduled as something that we are going to do. At least not yet.