A deviation in the purchase price can have the following reason:

In the case of prorated purchase requests, the desired amount will not be exceeded, but may be slightly less. This is because prorated purchases are billed to a maximum of 3 decimal places.

As an example: The share price is €94.98 per unit/share. You would like to invest 5 €, calculated from the purchase price, this corresponds to 0.052 units, i.e. 4.94 €. At 0.053 units, €5.03 would have been billed. This amount would exceed your desired buy order and so the unit below has been chosen.