Why are some miracles hard to explain and some are not? Here is a related question but they are not the same
Do miracles violate the laws of physics?

Looking at most miracles in the bible, they can be explained based on what we already understand (physics, chemistry, etc) and sometimes they were even explained in the bible. Like in the case of God parting the red sea, the east/west wind had to be strong to drive the water apart and made the children of God passed. There are others like that.

But there are 2 miracles that are difficult for me to explain. 


The Shunamite widow pouring out oil from nowhere (2 Kings 4) - Here is an analyses have read but not satisfied: http://www.jba.gr/Articles/nkjv_jbajuly97.htm
Jesus feeding 4000/5000 from food that did not exist


I know God is omnipotent and can do all things, but in the case of the Red Sea, we learned how He did it. 

My question is, when Jesus had 5 loaves of bread and 2 fish, how does breaking them increase them till they could feed 5000 people (I know it's a miracle)

I am even more concerned about the Shunamite woman. Having a jar of oil, and pouring into several jars and could have continue if there were more to fill. How did the oil keep pouring when there was a finite amount inside (diluted with water - I don't think so)
Any answer to this question will be pure speculation - the Bible doesn't say why there are these two "classes" (the question of the existence of said classes I'll leave alone for now), so we don't know. 

With that said, here are two potential reasons that are Scripturally based:


God is sovereign over everything, and is pleased to work out his will through his Creation, often utilizing secondary and tertiary causes. The Parting of the Red Sea shows this , and it is no less a miracle because God used one part of his creation to affect another - after all, God created it all in the first place! 
God hides things from us. There are things we cannot yet know. While God's power is manifest in the Red Sea, marveling us, his mystery and wisdom are manifest in the feeding of the 5000, keeping us grounded in our own frailty. 


That is, God demonstrates different attributes through different miracles that serve different purposes.