Those online Trading Services have a cap on trades up to and...

Those online Trading Services have a cap on trades up to and including a certain value eg Comsec, E-Trade. A % fee applies thereafter

Brokers can buy using a range of methods on your behalf eg algorithmic (or 'Bot') trading; i.e. multiple transactions to fill an order. The one trade you saw may have been the end of a fill, an attempt to cap a price for further buying later on etc. It's the robotic manipulatrion we hear about time after time and a disdvantage to the Mum and Dad traders/investors such as ourselves. This is my view but DYOR Cheers .... J