The answer to question 1 is very easy. If there is any mistake made, it is not made by your bitcoin QT wallet, because the wallets (actually the addresses) are the backbone of the network. The webiste blockchain.info only read information, and mistakes can be made.
Regarding your second question, it can have to do with change. Whenever you send a transaction and the total input value is more then the amount you want to send, the wallet automatically creates a change wallet, where the rest of the input is sent to.
blockchair reads the same amount as blockchain.info.
Right so when i send a transaction i've started to tick the box to minus the transaction fee from my transaction, is this why my transaction creates a change wallet ? Sometimes the amounts going into the second wallet are considerably more than i am sending - is this right? im not hacked or anything here?
So let me just confirm with you because i sent like 0.1 from my addy to exchange and apparently blockchain .info says i've sent 0.5 to someone else all included - Bitcoin QT has no history of this ofcourse like i said before.
Many thanks for any help understanding.
I am not sure if I understand you correctly, but this is how it should go:
For example, you have 1 BTC in your address A.
You want to send 0.3 BTC to your friend with address B. The 0.3BTC goes to your friend, but the 0.7 BTC goes to your change address C. Whenever you create the transaction, it should look somewhat like this:
Address A -1BTC. -> Address B + 0.3BTC & Address C + 0.7 BTC.
Can you maybe post the transaction ID, so that I (and others on this forum) can look into it.