Re: trying to send a fraction of a btc, 'preview tx' button indicates otherwise
This sounds normal to me. Bitcoin doesn't work like a database usually would.
Simplifying but: if you have 2BTC in one lump (UTXO) and want to send 0.5 BTC somewhere, it effectively splits that into 1.5BTC (your change) and 0.5BTC. The .5 gets sent out to the address you specified and the 1.5 gets sent to a change address that is part of your wallet.
This isn't unique to Armory, so there is plenty more reading available
Simplifying but: if you have 2BTC in one lump (UTXO) and want to send 0.5 BTC somewhere, it effectively splits that into 1.5BTC (your change) and 0.5BTC. The .5 gets sent out to the address you specified and the 1.5 gets sent to a change address that is part of your wallet.
This isn't unique to Armory, so there is plenty more reading available