Reason for difference in behaviour of autcomplete interactions in input fields

There seems to be two different schools of thought when it comes to the autocompletion of input fields and I am wondering if there is a particular reason or rationale for each.

Basically, when the user is completing an input field (usually text based) it is typical for autocomplete functionalities to be implemented when there has been previous inputs provided. The user can then choose to continue completing the input or to use the autocomplete feature by:

• Pressing the ENTER key
• Pressing the TAB key

Alternatively they can use the mouse and click on the autocomplete suggestion but it breaks the flow of typing from the keyboard.

Is there any particular reason (both from a technical AND user perspective) why some sites use the ENTER key while others use the TAB key for this? I think it creates a confusion because it is likely for a user to go to websites that implement this feature both ways.