Why are my apple earphones and earspeaker working at the same time?

This is a weird glitch or something. When I put my apple earbuds in my iphone 6s, my left earbud doesn’t sound as loud as the right one. Then I found out when I have my earbuds in my iphone, the calling speaker is also working. So the calling speaker takes like half of the sound of my left earbud. Does anyone have an answer?