Can someone explain why one Smart Value works and the other doesn't (list processing question)

[Derek Fields (RightStar)]

I spent several hours trying to add the "Amount" field (float) from a list of Asset objects.

The following Smart Value provides the correct answer:

{{lookupObjects.Amount.join(":").split(":").sum}}

The following Smart value provides nothing 

{{lookupObjects.Amount.sum}} 

lookupObjects.Amount returns a list, which I join and then split - then the "sum" function works. 

I would like to understand what is underlying the Asset object so that I can save hours the next time I am trying to do list processing on this.