It depends on how your mean is calculated, so this is my question to you, is it possible it doesn't rise after a won game or even decrease?
If we're talking about 'basic TrueSkill', which is what I will call the version described by the whitepaper on it, then I'm pretty sure that a win always increases your mean.
However, I'm not sure if WL uses 'basic TrueSkill' or not. And it may be the case that the RT ladder and the 1v1 ladder use different tweaks to TrueSkill.
For example, it's common for your TS rating to change in the 1v1 ladder even if you haven't finished any games recently. I believe (not sure) that this is because the 1v1 rating system recalculates everyone's ratings all at once periodically (like once a day or so, it seems). So, if someone you played recently had their rating changed by a recent game of theirs, then that might affect your rating indirectly. Also, since old games can expire, this can also affect your rating.
I know much less about the RT rating system. I've heard that it doesn't expire old games. That makes me suspect that it *could* use a more simple TrueSkill system, perhaps 'basic TrueSkill'. If that's the case, then I think a win would always increase your mean. But I don't know for sure.
I could be wrong on any of this stuff, so take it with a grain of salt.
I will say this, though: If there is a version of a TrueSkill system where a win can actually lower your mean, I would call that system 'broken'.