Did you read the links? specifically the Coin Analogy one?
https://en.bitcoin.it/wiki/Coin_analogyWhen you receive bitcoins in multiple transactions, they don't all merge into one balance, like they would with a bank account. They remain as separate pieces. For instance, say you received:
0.1 BTC
1.0 BTC
0.7 BTC
You now have THREE coins, totalling a value of 1.8 BTC. If you now decide you want to send out 0.012, your wallet will generally (but not always) use the smallest coin it can, that covers the total to send + transaction fee.
So, your wallet picks up the 0.1 BTC coin, and uses that... breaks off a 0.012 piece and a small bit for a fee (0.000853)... and then puts the rest (0.087147 BTC) back in your wallet as change.
You now have:
0.087147 BTC
1.0 BTC
0.7 BTC
If you now wanted to send 0.1... your wallet would mostly likely use the 0.7 BTC coin etc etc...
So, in answer to your question:
But I wonder where the 0.1 number came from? I have 1.8 BTC in that account address I sent from..
At some point, you would have received a transaction that sent 0.1 BTC to your address (or wallet)... so you have a coin worth 0.1 BTC sitting in your wallet, which your wallet decided to use to send out the 0.012.