Why sentir in the subjunctive?

Hola Shirley

"sientan" is from verb "sentar", not "sentir" (to feel) 

Sentar is also used when we want to talk about food or drinks and how they "sit" in our stomach. 

See here this specific meaning from wordreference. Look at the 5th entry with context "comida". 

So, in the present indicative this is: siento, sientas, sienta, sentamos, sentáis, sientan 

The verb sentar or sentarse (to sit, in general) and its conjugations are very similar to sentir (to feel), maybe the confusion comes from there?

Look now at the present indicative conjugations of sentir:

siento, sientes, siente, sentimos, sentís, sienten

I hope it clarifies it.

Saludos

Inma