Sure, I can help you understand! When you defecate, your large intestine contracts to move waste out of your body. As your large intestine contracts, it can put pressure on your bladder, which can cause you to urinate at the same time as having a bowel movement. This is a natural phenomenon known as "defecation-induced micturition," and it is entirely normal.
In contrast, when you only urinate, there is no pressure on your large intestine, so you don't have a bowel movement at the same time. Therefore, you only experience urination and not defecation.
I hope this helps you understand why you experience urination and defecation differently. Let me know if you have any other questions, and I'll be happy to help!