Sum Vs AGG

Hi everyone! I have a question regarding the use of SUM in a calculated field.   For the picture above, the GM(gross margin)% is calculated using [Gross Profit]/[Revenue]. Whereas for the picture below, GM% is calculated using SUM([Gross Profit])/SUM([Revenue]).   My question is, why did the calculation in the first picture not work (what exactly was it calculating)? AND why do we need to use SUM? Thank you in advance!