Difference Between SUM & '+': Formula Field Works One of the Time

I have a very simple formula that is only returning one result.

[Travel Charge]+[Labor Charge]+[Expenses Total]

The Travel Charge and Labor Charge fields are Rollups and Expenses is a Formula field.

Of course, as I was compiling this post, an idea came that I should try:

SUM([Travel Charge],[Labor Charge],[Expenses Total]) and it worked! All totals are filled in. I decided to post this anyway to learn (hopefully) the difference between the two methods & why the first returned 1 result of 8. ???