Probably because it's not actually soldered, but screwed on. The band of metal is held in place around the coin by a small screw passing through the top. If the screw is too loose, then the coin can move. You might need special jeweller's tools to adjust the screw.
Such a holder causes the least amount of damage to a coin, but it will still damage the coin. So don't take the mount off; since the coin itself is not worth very much to collectors, it is now worth more as jewellery than a damaged coin. You might, however, try to loosed the mount further so you can rotate the coin properly, then tighten it again.
Regarding the silver fineness: the official fineness was .833, and this was adhered to by almost everyone who made trade replicas of them; Rome mint examples are .835, for example. This fineness is lower than that of American (.900) or Canadian (.925) circulating silver coins.
Oh, and here's the Wikipedia page for the Maria Theresa thaler
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis