It might be anything - some people prefer German uniforms or weaponry or they like to be "the bad ones" (Republic was always outnumbered by the Sith Empire). On russian servers people overstack Soviet's side. National pride, I guess.
I always try to join the attacking team first, regardless of what nation this is, then any with a free slot. I agree that German team is probably overstacked on european side (where I play mostly), but this alone doesn't mean Germans always win. I've seen some very good Soviet teams.
In fact I saw that many times people are leaving the side which is... losing. This creates a constant loop: losing players leave so they open slot for other people who join the losing team and so the fallacy of "unbeatenable" German Team is created.
My favourite team is Allies (because I have to join Soviet team as Germans are overstacked most of the time and a few other factors) and my win-to-lose ratio is 0.87. 401 wins to 451 loses. In short: no, Axis - Germany in particular - not always wins.
Some outcomes can be attributed to maps, which can be seen clearly when you play the campaign mode: some maps aren't attacked most of the time and when they are chosen the defenders quickly show how big mistake this was.
I disagree for the RS part. The US weapons are NOT better. Besides the flamethrower, all other weapons are not giving any REAL advantage over the japs in like 99% of the maps.
Say that to an American - with M1 Garand - in his face when you have to bolt your weapon... Weapon capable of taking a few shots in a row is clearly superior to a weapon which needs to be bolted after each shoot. Japanese players have to make up their firepower disadvantage by other means, but saying that US weapons are NOT better isn't true.
We're not talking IS (some prefer DP-29 over MG-34). We're not talking about ammo capacity (where DP-29 has lesser magazine than MG-34). We're talking about how many rounds you can eject while aiming at one another. Unless you hit and kill you're going to be at a disadvantage.
Commissar's House, Red October Factory and Apartments are perfect examples on how to bias a map towards one team. Apparently, it's the Axis team. None of these 3 maps requires balanced teams. It's enough if the Axis team has 5 capable guys that know what to do and can convince another 5 guys to help them out on blocking choke points or calling in Artillery at the right time.
I remember when the German team was forced to attack though the Red October Factory. We - the Soviet team - simply slaughtered them. They weren't even able to get past the first objective, although they made a few brave attempts to capture it. You simply can't win against a team of battle-hardened veterans who have been playing this map from the attacking side's perspective over and over... I, myself, took down many Germans with my trusty DP-29. Some maps are simply very hard to attack, regardless the side, often experience in its defense/attack also matters.
That's what I like campaign mode for - it can swap sides of attackers and defenders for certain maps. The other side has the taste of that the "usual attacker/defender" has to do in order to win it and then it ain't that easy.
Honestly, maybe TWI should just get rid of the German and Russian skins and replace them with solid block colors. Then they should just get Microsoft Sam to do all the voice overs.
You'd be suprised how sane people would be then. Just think about this like that: do you want to play as red or green block? Or you want Empire or Republic? When there is nothing to attribute, there is much less ideology involved and less fuss over it when the main difference between block A and block B is its color.