Traditional Pottery Making of Kalinga 

ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Qu9l4roeGnc.html (my pottery designs)

I think its for the softness of the top so it will not be rough, maybe.

Pretty sure it's a waxy/resin sealant to keep the pots shiny and nonporous....if they are pitted and porous they can hold on to bacteria.

its a type of resin wood that are used a lot in south east asia, it is used to seal the pot and also to seal ships and boat in boat buildings in south east asia, i dont know what they call in cambodia but in my country we call it DAMAR. Its like lacquer i think

I can only guess, that was what people simply observed in nature. Round egg, coconut shells, etc, good container. (Actually science learnedthat best energy containment with respect to strenght is sphere shape, and egg shape has good circulation) Hence maybe that what they also observed, that round/ spherical is stronger. Also easier to clean.

every vessel that are rounded bottom are stronger than flat bottom if made of clay.

Salt ant pepper

i think its a very very old song of the people. i think the song is about a thousand years ago.