Xp gains for kills?

So let me start by saying ive played this game for 8 or so hours now and i have no xp perks/skills. Anyways i was leveling up decently in the beginning, then it slowed down drastically about 3 hours in. I did some investigating and found out when my weapon of choice (suppressed scoped hunting rifle) 1 shot killed enemy runners (dogs?) by hitting the component on the back I earned 9xp for that kill, the entering combat warning always goes off even though its a stealth kill and theres no further engagement and im not seen. Switching to my .32 suppressed pistol i go for another single runner (dog) firing many more rounds, being seen, and sometimes getting shot killing that enemy to receive 39xp. More than 4x that of the hunting rifle. is this happening to anyone else? Is there a dynamic to earning experience that im missing? Does anyone know if this distribution of xp is intended? Thank you for your time.