Why does Tiger fill my order at a slightly higher price than what i agreed to pay?

Yup, it's the platform and commission fees that were added. Since the fees are added to your total, the more shares you buy the less the increase will be per share cause it is divided between all the shares

Hmm, check under your order detaila. Perhaps it's factoring in the comms/ platform fees? ie USD1.99 of comms and platform fee would translate to a ~USD1 increase in purchase price