I could explain it better if I saw the replay, but I'll do my best to explain.
Experience has a radius, regardless of you killing a lane minion or not, you get EXP from them when they die. Melee minions offer 59 exp, casters offer 29 exp, and siege offers 92 exp. When I went into the practice mode and added an allied target dummy, I started getting 38 exp from melee, 19 exp from casters, and 60 exp from siege.
In bot lane, you need the first wave (3 melee and 3 casters) and the 3 melee minions from the second wave for both to get level 2 ( (6 x 38) + (3 x 19) = 285 EXP total, and 280 EXP is needed for level 2).
In mid or top lane, you need the first wave (3 melee and 3 casters) and any minion from the second wave ( (3 x 59) + (3 x 29) = 264; 264 + 59 = 323 OR 264 + 29 = 293).
I'm assuming you started Blue buff with your bot lane leashing, took Gromp, took Red, and then ganked based on your CS numbers and damage done.
Considering it's a Garen, he probably started using his spin to push the wave at level 2, hence why you went up to gank him. Because of that, he lowered the minions health quickly, but possibly didn't get the last hit because his spin doesn't do much damage. He might've even used his E during the gank, killing minions as he tried to escape which would net him more EXP when they died. Since he died on your side of the map, he might've even pushed the minion wave to your tower, which means he probably had three full minion waves of EXP while Darius might've only had two with a full minion wave sitting there waiting for him to kill them.
Additionally, since you were in the lane, you were leeching experience that Darius would've gotten, putting Darius slightly behind. He got some additional EXP from the assist, but assuming he used his pull and Q during the gank, he might've accidentally killed some minions while you were there, forcing you two to split EXP.
Honestly, go into your match history, download the game replay, and watch it. You can see how many minions die for each team and see why Garen has more EXP than Darius despite Darius having better CS.
has 1 assist and 8 cs, yet is still on level 2, whereas 
has just 6 cs and one death yet is already at level 3. How is this possible?
