Why is my ticket so much cheaper on Trip.com

Hello everyone. I'm a rookie traveler, never have really traveled anywhere even domestically. I decided to buy a ticket about a month ago to the motherland in Vietnam. I got it through Trip.com for $745 from Lax to Ho Chi Minh. It looks like after I land in Singapore from Lax, I will be taking a flight on Scoot plane to Ho Chi Minh instead. From what I know, Scoot is basically Singapore Air little sister and won't be as comfortable or have the same service but I could care less cause it's only a 2 hours flight. I'm just curious is that why it's so much cheaper? Cause I just look on SingaporeAir site and tickets are going for 1,100 right now. I'm just a little bit paranoid since it's my first oversea flight. And Is 1h20 enough time for me to get to Scoot boarding zone after I land in Singapore? Thanks in advance!