Minions Last hitting and Gold Bug - Mac 10.9

Played 3 games. 1st game ARAM did not happened or maybe i just didn't notice. 2nd game Rift this happened and i relogged to make sure it wasn't some silly anomaly. 3rd game again... so here's what happened and hope this makes sense.

Farming bot lane, i see a few minions showing gold, the gold +1 that shows as if i had last hit them, but i didn't always last hit. they are not showing that i killed them in my minion calculation. the money doesn't add to my account either, but then I notice it also happens when my minions kill enemy minions. i just tried a custom and it is also happening. Now it is very confusing to see if i actually last hit them or a minion killed the enemy minion and the coin sound sure makes it sound like you are doing awesome. however it is not the case. :(