Incoming vs sent text tones

I’m sure someone knows the answer to this mystery. On my iPhone SE, I have the text tone set to the default tone, the familiar “ping” sound. When I’m in a conversation with someone and I receive an incoming message, I sometimes hear the “ping” but most of the time I hear the message sent sound - that short “zoop” sound. Anyone know why? I’m wondering if Apple set it this way because hearing the “ping” sound all the time might be annoying if you’re exchanging lots of messages in a short time period. I bring it up because I sometimes miss incoming messages due to that quieter sound.